Hi, i’m not too far in the game, i’m kind of at level 4 soldiers (i restart 4 or 5 times my runs because i realize i made wrong choices), so i don’t know how the game evolve in term of recruitment.
My question is: is it better to recruit the more level 1 soldiers i can and make them level up in my base (without use them in missions), or am i going to find soldiers to recruit, later in game, but with higher levels?
Both. Early on you will soldiers. Training will level them up, but one doesn’t get much EXP. Take 1 or 2 out on missions to get EXP. Later on you will find some higher levels to recruit from havens.
EDIT: Technically, it\s SP that one needs from missions to spend on skills.
Perfect! Thanks! 
Building 2 or 3 training buildings can speed up training, so if you have e.g. 2 of them in a base, dont hesitate waiting - its a long war, and as said, by combining in base training and taking them as 2nd squad, you can train them better then by waiting for fractions to pop up with some super soldier. Surely, late on, you will see mutated soldiers (Annu) and soldiers with more ablities in heavens too.
However, I find recruiting more expensive but far better option, since on Rookie and Veteran you get quite equipped soldiers, and if you get 2 of same class, you can quite reverse engeener armor and weapons, getting a speedy access to better weapons and armour to long alliance route or spoiling relations by stealing. On harder difficulties since you would get “naked” soldiers, I often prefer to train and equip on my own - at least base price in food only saves much needed resources.
I only understand today what you wrote!
So, if it’s like that, is it a good strategy and viable, to keep a base only to put the maximum of training facilities (4,5,6,7?) and put all our rookies inside? (It’s like a bug or an exploit or is it really intended? 
)
EDIT: i really need to know that, because if it’s a bug or an exploit, i don’t want to use that, it will ruin all the fun i have to play PP. (if it’s intended, ok)
It’s not a bug. However, they are expensive. 2 or 3 should suffice. They will still need to go on missions to get the EXP in order to get skills. Some will see it as an exploit.
Thanks! So i can play peacefully… Training facilities, i’m cooomiiiing! 
As told, its not a bug. When building it, it tells you exactly how many skill points per day I believe each give you - just like having 2 medical bays doubles medical recovery time, having 2 living quarters speeds up stamina recovery time …
However, it only gives skill points, to improve fully they need combat experience so they level up / in other words you will have a lot skill points, but you will need combat experience to unlock all options of skills.
This is IMO not entirely true.
You need to differentiate between experience points and skill points:
Experience points are only there to level up and nothing more.
A unit can gain experience points in missions at specific rates dependant on how they perform or in bases with training centre(s) at a fixed rate per day (I don’t know exactly how much, but I’m pretty sure it increases linear to the number of TC per base).
Skill points (SP) are the ones that you can spend to activate skills or increase attributes.
A unit gains skill points (SP) when it levels up with 20 SP per level and by finishing missions successfully with a fix rate per mission dependant on the difficulty level (rooky = most to legendary = least).
That said, with TC you can get level 7 units for free (only spend time and food per day), but they will have not more than 120 SP to spend (20 SP per lvl, from lvl 1 to 7 → 20 SP x 6 = 120).
With “training in business” they gain the same SP per level up (120 SP up to lvl 7) but additionally also the fix SP per missions, so you end up with much more powerful soldiers, but of course with a hard way to go for them.
Edit: tried to make the explanations a bit better … hopefully successful 
And you know if XP is given to a unit for an opening of a chest, a kill, an assist? (because i wondered that yes and i liked to choose, this or this one to make him level up faster than otherd)
They get a good amount of XP for kills, that’s pretty sure. But all the other things … well, here I’m not sure, maybe some XP, but I assume not that much.
I almost try to take a middle route, so that rookies train XP for leveling mostly on bases with TC, but I also take them to missions mixed with more experienced soldiers just to get some SP and these are not dependent on how they perform. Every soldier in a mission get the same amount of SP (on veteran 12 per soldier and that is pretty good IMO). The veterans almost do the main job but these rookies were mostly not really useless and when I see a chance to kill an enemy without risk then I give it to them.
About XPs vs SPs:
This part is nearly finished, there are just a couple of things I’m not sure of:
If anyone can shed any light on this, it would be much appreciated 
Hmmmmmm, it seems we have a save scummer here… 
I have failed missions, but I have conflicting memories on what happens.
Like I seem to remember that when I failed a steal aircraft mission (I have been experimenting with raiding and sabotaging) because the aircraft was destroyed, that all my guys got their SPs.
But I may be wrong, thus the query.
I think they get individual SP, but 0 for the pool.
Sure… 
As I observe, unit that shoots, get wounded or completes the mission goals gets additional XP
So yes, actions get rewarded.
For both points I can’t see a rule. Of course there will be one, its a program and I can’t believe it will be randomly calculated, I just can’t get it right.
Mostly you don’t get SP if the mission fails, but on some failed missions you got them. For instance resource raids, but I think they not really “fail” if you simply evacuate in your first turn (yes, this is endless exploitable as long as you don’t bother the reputation loss).
To the common pool SP, it often seems to be a fixed rate of total XP. But as mentioned above, I can’t figure out the pattern because it is slightly different from mission to mission and sometimes quite far from the usual rate.
The thing is they get rewarded at the expense of their teammates. The total amount of XPs per mission is fixed, individual performance only impacts distribution. Say a mission gives 400 XPs, and you brought along 4 operatives. Assume that your operatives do nothing (and somehow the mission is still a success): each will get 100 XPs. However, if one of your operative does many things and the rest do anything, he will get say, 205 XPs, while each of his team mates get only 65XPs.
This becomes important when you bring LVL7 operatives along with lower level soldiers: the LVL7s will still get XPs, and if they do a lot of things, they will eat more of the XP pie. However, they can’t advance beyond LVL7, so those XPs will be wasted on them.
My working theory is that the total amount of XPs for mission depends on difficulty, with low difficulty missions giving 300XPs, and extreme 900XPs, and the SPs that go to common pool is maximum XPs that can be awarded per mission/100.
However, as you say, every once in a while I find exceptions to this rule - like missions that give 500XPs, and 9 SPs.