Sounds like you have internal knowledge of this process ... how does it work, do you have to formally tell your boss that your fridays will be spent on 20% time? do they need to give you permission? or do you just have to sneak the time in at your discretion?
The normal way it works is you accept a 20% role from another team. Teams post 20% opportunities in the same way they post transfer opportunities internally. You talk to the hiring manager and see if they'd like to "hire" you.
A common reason people do 20% work is to test the waters of another area of the company. It's also a kind of tryout for you.
You do tell your manager in this case, yes. They're not supposed to say no. Hard to say what the range of responses is in practice. In my experience managers are supportive of this.
