Is there a tangible difference between source first / source available and open source? Afaik if you are just a normal developer and not a company it's pretty much the same?
Yes.
Source First: https://gitlab.futo.org/alex/latinime/-/blob/master/LICENSE....
Open Source: https://opensource.org/osd
Notably, Source First does not allow for the software to be used commercially ("You may use or modify the software only for non-commercial purposes...") whereas Open Source does allow commercial use ("The license shall not restrict any party from selling or giving away the software..." and "The license must not restrict anyone from making use of the program in a specific field of endeavor.").
Microsoft shares Windows codebase with its partners. Is there a tangible difference between it and Android?
Also, as a developer, why would you volunteer for a "source first" codebase? I don't see any point since you can't really fork it, in the traditional sense.
Only a limited number of people can view the Windows code and nobody can make their own version of Windows.