Best Practise for users who have left the business

Hi, I have never really got involved in User Admin, more CRM development.  But now, I need to advise a company on the best practise for users who have left the business, and having their licenses available to be transferred to new users.

I'm not sure of the exact steps, is it something like:

1. Disable 365 Account
2. Disable CRM account

Will disabling a CRM account free up the users licenses, or does this need to be done manually?

Also, is it better to delete the 365 account?  If so, will doing this affect CRM?

TIA