If you choose File > Save as Default, the current settings will apply to any new album project you create. That includes the choice of skin.
If you choose File > Save as Skin Default, the current settings will apply to any new album project you create with the currently-selected skin. So, you can save defaults for Tiger, defaults for Saturn, defaults for Minimal, and so on.
These settings are currently saved to your configuration directory. That's a fixed location at present, and not easily changeable.
I have ticked Facebook in the Social>Share On settings, but it doesn't display when Share is selected, all the others I selected are shown...
That's skin-specific, and I don't generally "do" the social media stuff (not included in any of my own skins), but if it's Tiger, the Facebook share will show at the album level, but not for individual images. I believe that's because Facebook has made it virtually impossible to share at the image level.
And BTW, not stupid questions at all. There's a lot of stuff buried in the settings and preferences. There are some choices that are truly tough to find. Profile page privacy settings, for example, don't appear until you're actually in the process of doing an upload.
Even after a decade, I still sometimes have to hunt around a little for something that I'm sure is there, somewhere.