Questions about ERC

Hello, I have been actively studying the ethereum blockchain for a while and I keep getting ahead of myself with all the directions people can take. I see an exhaustive list on github with all the known major token standards (there seems to be like 30 differents kinds). But no matter where I search, there are a few things I can't seem to find. I wonder if someone can clear a few things up for me or maybe explain to me where my understanding is incorrect. I would also love some pointers on places to learn more about ethereum or places to learn about blockchain technology in general. Thank you!

1. I understand that these standards were made to simplify the process of a token being accepted onto an exchange so that it can be swapped with other tokens. Can you make a token with a contract that isn't based on one of these "standards?"
2. If so, how hard would it be to get such a token on an exchange? I can't imagine an exchange would charge so much if all they had to do was code a little bit to accept your token, right?
3. I hear that smart contracts, once made, cannot be changed. But I see things like erc1067 (upgradeable) or multidimensional tokens that seem to have the ability to add more functionality for your token even after you deploy it.