It’s true we don’t currently have good user documentation for Plone 6 but we are working on it.
Interesting question, how to change the admin password! You do that by going to the back end, e.g. localhost:8080/manage then acl_users (i don’t have it in front of me to give you a more specific answer) because admin is a Zope user, not a Plone user.
To put a blog on the internet, we have several tutorials though they’re in need of updating. I posted one here earlier … I will find it
It depends on what kind of customization you mean. Simple things are settings you can change. Others require installation of a Volto theme or addon. Others require development. Doing that with a Docker setup is more complicated. Maybe others can answer this question better.