In real practice what are the standards I should definetely be familiar with?
Familiar == can explain what functions such a contract needs to have and what purpose it serves.

According to my research ERC20, ERC721, ERC777, ERC1155 are the ones I should learn by heart.
What else?

Making sure I am not missing something important.

Ok. Thank you, I'll look into it.

Sorry for a dumb question, I just want to clarify it for myself.

Let's assume I have 2 mappings:

mapping (uint => address) testMapping1;
mapping (uint => address) public testMapping2;

If there's no visibility modifier specified for testMapping1, how would it behave?
Is it a good practice to always explicitly set visibility?

Never mind.
Found the answer myself: https://docs.soliditylang.org/en/develop/contracts.html#visibility-and-getters

internal

Internal state variables can only be accessed from within the contract they are defined in and in derived contracts. They cannot be accessed externally. This is the default visibility level for state variables.