What's the main difference between "harassment" and "abuse"?

What does "give [someone] the finger" mean? In what circumstances could I use it?

To "give someone the finger" means to show someone your middle finger, which is a very rude hand gesture that means something like "fuck you." You can use it when you want to say that someone gave someone the middle finger. Ex: The driver gave the finger to the person that cut him off at the intersection. Ex: If you give someone the finger, you may get cursed at.

What's the main difference between "harassment" and "abuse"?

Harassment causes stress or worry to a person by persisting or crossing lines, whereas "abuse" is the unjust and bad treatment of someone, physical or non-physical, or ill-use of power. Depending on the context, harassment can either be a form of abuse or can lead to it. Ex: I was tired of being harassed by the phone company calling me every day. Ex: Just because you're bigger than her doesn't mean you should hit her. That would be abuse. Ex: Abandonment is a form of emotional abuse and can take time to heal from it. Ex: I was harassed walking home from work again. I'll take the bus next time.

Could you explain the meaning of "make it" here?

"Make it" here means "arrive" or "arrive safely". Here are some examples of "make it" with the same meaning: Ex: Did you make it to the airport? Ex: I barely made it to work on time.

What is "shoulda"?

"Shoulda" is slang for "should have" or "should've". Ex: I shoulda done my laundry yesterday, now I don't have clean clothes.

Does "they" here refer to "trees"?

Yes, your guess is likely correct. The speaker means to say "just like how it is done in nature," which is likely referring to the trees they have just planted. Ex: Just like how trees do it in nature. Ex: I only make authentic pizza with no weird toppings. Just like how they do it in Italy.