How does per user licensing work?

Hello,

I need to get a silver plan for my company and I would like to know how does per user licensing work exactly.
Is it per group, per plan, per project or it’s just through all projects and groups?
Like, if I 3 groups, do I need 3 plans? Or just 1 plan for 3 groups? Can I use the plan on two groups and not on the third one?

Do CI minutes add up for each user or is the same no matter the amount of users?

How about our open source public projects and repositories? If I add users to these repos, does that force me to pay for the users I add to these public repositories?

Thanks,

Mike