Help me understand the game more

Basically what the title says. I have a few questions and was wondering if someone could answer them as simple as possible for me. I love pokemon and it has a special meaning for me but I honestly don't have a good range of knowledge of certain things pertaining pokemon and didn't know if it would help with this game any or not. Anyway my questions are:

  1. Do the nature's in this game mean anything and if so what do they do? (Don't have to explain every nature just the gist of things)

  2. Is having many poke friends better than having a variety of them? Like is it better to have multiple of same one or better to have different ones.

  3. Levels. What they do and why do you need to do it. Also, again, better to do on multiple of same or variety of pokemon. And is it better to have more of a group you just focus on til a certain level.

  4. What should I spend my diamonds and stuff on? Basically rn I'm not spending anything because idk what's worth it.

  5. Last question, sorry. Feeding snorlax throughout the day worth it? What does leveling him up do or mean really?

Basically I wanna enjoy and get the most out of this game but rn I'm feeling so confused and lost that I feel like I'm just not doing things right or like I'll mess something up soon and regret it. Any advice is very welcomed and helpful, so please let me know! Thank you so much in advance!