Where are admin theme css classes?

https://codex.www.ads-software.com/Creating_Admin_Themes
I believe that will answer all of your questions.
and just for luck https://www.google.com/search?q=wordpress+admin+css

I wish they did. All the info I can find seems to focus on how you can add you own custom styles. Not how to utilise standard admin styles. How do you set up forms? Is there a grid? Etc. etc.

There has to be some kind of layout built into WordPress, I don’t need to create a custom theme.

Do you mean adding tabs and options to the admin section or just styling it?

https://developer.www.ads-software.com/themes/
https://developer.www.ads-software.com/reference/functions/add_menu_page/
https://developer.www.ads-software.com/reference/functions/add_submenu_page/
https://developer.www.ads-software.com/reference/functions/add_settings_section/
https://developer.www.ads-software.com/reference/functions/add_settings_field/
https://developer.www.ads-software.com/reference/functions/register_setting/

Hope this has pointed you in the right direction.