Should i start with a theme? Plugin? MultiSite? How to do a multisite?… Where to start.

Good day, I’m a beginner so this is a beginner question. Maybe a whopper of a beginner question though… I’m developing a website with multiple sections all to be on one domain: . Here is a screenshot i took of a little flowchart i’ve started working on to give an idea of what i’m trying to do with it: . I think my question is going to largely be about multisites on WP. I just don’t know where to start. Themes or plugins? How to set up a multisite if that’s what you recommend? Are there any tutorials i should be watching or anything before contacting you?