I am a full time author. (I'm learning graphic design to better present my books.)
I've been using social media to promote my books for years. I noticed that it is more important for me to be a member of Facebook bookgroups (in my genre) than to have a lot of friends there.
On Instagram I'm in a "book bubble". I'm not very active there, but I check it out regularly. When other authors or book bloggers are liking one of my posts, I like back. I do not search active for friends anymore. But I try to be polite and cooperative.
Everyone wants to present themselves on social media. The art is realizing what you can do for others, not what they are doing for you. Give something and you get something in return occasionally.
On Bookstagram the bookreaders and authors often share books or interesting posts from others in their story. For this, you return the favor at the right opportunity.
Be aware, that only 5 to 10 percent of the people who offer you social media friendship are even interested in seeing your work. Most people want you to look and like their artwork. They can be valuable as part of your network, but don't expect too much from them.
Don't automatically like everything that is shown to you in the Instagram feed in the hope that you will get likes back. That can go wrong terrible if you accidentally like a post that is nasty, racist, or right-wing extremist.
And to answer your third question:
My brain is trained to produce novels. I write several hours four days a week. I have too many ideas. I think it's the same with graphic designers and illustrators. As an illustrator, I'm just starting out. I worry a little that I don't care about making fanart. That may be a flaw in my personality. I prefer to paint my novel characters. (Not that I already can.)
Put pictures in your mind. So look at a lot of pictures by other artists, classic and modern. The more you look at, the wider your spectrum of ideas becomes.