Examples of saving information from a new form into database, node or config entity

Problem/Motivation

I tried to use 'examples' to build my custom form but I ended up using this video on YouTube. I don't know if it is naming of files or something else but it was unclear to me which example I should use.

Also, I haven't found any examples of how to save data anywhere. I can only see examples of outputting values into Messanger.

I suggest adding some examples of saving form values into files/database. There is no much info about this on Google, only some old posts like this one: how-can-i-save-form-data-on-a-node-in-drupal