in oracle 21c there is no mention of DGA and what it does, I couldn't find anything anywhere else.
what is DGA? and what are it's uses ?
It is the "dynamic global area". It's a pool of variables that are initialized when the PGA initializes and are not changeable. Putting them in a shared space allows all your database processes to access them, like global variables. I think these variables must be initialized prior to SGA initialization, which is perhaps why they're in PGA and not in SGA.
I don't think you can use DGA in your application code, nor do I think you can tune its size. I think only Oracle features can use it.
This is my understanding based on very limited information that's available.