I always get mysqli_sql_exception #2002

"php spark serve" uses a different php.ini than your MAMP, so it may not have mysqli or other settings enabled. "php spark serve" are the fast way of getting things up and running, but you can use either of them.

If you choose to go the MAMP path, you need to create a virtual host and point it into your public folder. You should never do:
http://localhost/codeigniter4/public to run your app.

"No such file or directory" means your MySQL server don't have a correct socket path.