noob question about register settings

been doing a bit of codex reading, I understand that we can add, update and or delete options using functions like add_option or update_option, but many tutorials out there recommend using the settings API which is rather confusing to me… So, here’s my questions

what is the $option_group in register_settings() used for?

Can I specify any value for the $sanitize_callback parameter or does wordpress have a list of functions I can use to sanitize the option?