Some Questions about Blog Layout - Please Help

So, if I choose Blog or Blog Post, category stays greyed out. I have 2 categories on my site: Forums and now Blog. Was I supposed to make Blog a sub-category with Forum as the parent?
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Still a little confused on the theme thing. Is my entire forums site supposed to be Blog Landing theme, or only on the plugin? Like, if I want to use my modified version of Air, can I keep Air as my forums theme and then just set Blog Landing theme in plugin settings? I’d hate to have to re-make my entire theme.

I also can’t seem to figure out how to make “forums.mysite.com” the landing page. I would like if people got to the landing page by visiting my forums URL. I also can’t change my path at all?
https://i.ibb.co/j87712R/kkkk.png

Lastly (for now), how do I make the blog landing page URL the default page people get to when they visit my forums?