I still don't know how really the game works, so I take everything i see. Do i really have to ?
No, you do not have to take everything you see.
Personally, I ignore everything except magical items (Stuff with a different coloured name) and Supply Packs.
If i do not, why can we pick skulls, empty bottles and barrels ?
It's possible to use these things. Random junk can be thrown at enemies (Although it does not do much damage). As well as it can be sold to make a small amount of money.
Barrels and Crates can be used to climb onto in order to access certain areas without using spells. You can also use them to do silly things like using them to prevent people from seeing you sneaking, or blocking a path so enemies have to run to a specific area to attack you.
As the money, I don't know what the use of it (if it's not for food). The 2 merchants I met doesnt sell anything useful (only jeweleries and clothes with no abilities) and never change their stock.
For the most part, many merchants will sell unique magical items which is the primary use of money. Beyond that you can use it to buy supplies like Scrolls, Potions and Food as you require.
How works the alchemy gameplay ? Do the resources regrowth after a time ?
Resources do not regrow at all. However, some merchants you'll encounter will sell ingredients, which you can purchase to make potions.
Merchants all refresh their stocks after you Long Rest allowing you to continuously purchase more supplies (Be it ingredients, potions, scrolls etc)
-Second, the combats are very hard to master, and to me for now, it's a luck based combat for the most of them (I have 35/40% chances to hit when i attack my ennemies, just... Why ? 🤔).
It sounds like you're using attacks that your characters do not have the stats for.
Most melee weapons require Strength in order to be accurate with, the exception are weapons that have the "Finesse" rating, which can use Dexterity instead. All Ranged weapons require Dexterity to use. While Spells require a stat that is dependent on the class (Druids, Clerics and Rangers require Wisdom. Paladins, Bards, Sorcerers and Warlocks require Charisma. Wizards require Intelligence)
Certain classes can change these rules. For example, a Monk can use any one handed weapon or staff using Dexterity instead of Strength and a Warlock who opts for Pact of the Chain has an ability that can make their melee weapon use Charisma.
If your stats for whatever method of attack are low, then you will find that your attacks will have low chances to hit. As such, it's best to try and maximize the stat that corresponds with your characters main way of attacking to ensure the highest chances to hit.
So when i lack spells slots or potions (btw how can i get or make potions??)
You can acquire potions by either purchasing them from merchants, or by crafting them.
In order to craft potions, you can go to the alchemy page (There's a tab in your inventory for it). First you will have to "Extract" from ingredients and then you will be able to make things using the resulting extracts.
How can i find food to have long rests ?
You can purchase food from merchants.
You can also find it throughout the world. Often you will find food in barrels and crates, especially those close to or inside houses. Any food item you see can be used to long rest.
In addition, it's also possible to create it. Druids and Rangers can access a spell called Goodberry which allows them to create food that can be used to Long Rest with.
Can i "farm" xp and money by re-doing some combats / zone / monsters ?
No. Nothing in the game respawns. Whenever you kill something or someone, they are dead permenantly for the rest of the game.
The only way to "Farm" xp is by taking actions to maximize the amount you gain from a particular source. For example, early in the game you can encounter some bandits arguing outside of a tomb. You can maximize the xp you gain from this encounter by talking to them and peacefully persuading them to leave which will grant you xp... Then before they disappear you can attack them and kill them for additional xp.
Such actions should be considered though, given the aforementioned permenant results. So it is best not to simply murder everyone for xp (There are however some people, such as those bandits, that are side characters that never show up again for the rest of the game)
Another method of maximizing xp, is to trigger as many inspirations as possible. Each "Background" has a set of actions that when performed will grant an inspiration providing the character with a small amount of xp (And providing an inspiration point to reroll a skill check). Figuring out these actions to trigger inspirations for characters will allow you to gain more xp, with the idea being that they are somewhat related to the overall background itself (I.e. The Soldier background, that Lae'zel has, will often be related to combat or military training. While a Folk Hero background like Wyll has, will be focused around helping people)