If I need to restrict access to certain databases? So I've just created a MySQL server. I also created some users only having specific access to certain databases, but do I need to restrict access to default databases to the users I've created?
If so, how? Should I just create a role to try to restrict this access?

I have been using this site as a reference:
https://www.oreilly.com/library/view/mysql-reference-manual/0596002653/ch04s02.html
Generally you should start by running mysql_secure_installation on any mysql production database servers to ensure the minimum security measures are in place. You shouldn't need additional restrictions on general users after this.