What is it like being demigirl/demigirl? How much of a connection does one have to feel to their assigned gender to identify as demigirl/boy?
From what I've seen, demigirl is usually used by either trans people who haven't figured themselves out or cis girls who don't understand that it's normal for cis people to feel indifferent to their gender. I'd recommend not thinking of gender as a feeling, and think about how you want to be socially/biologically. If you actually feel a disconnect from your birth sex (and consider why to make sure it's not sexism) your probably trans and should consider how you're dysphoria makes you want to be.
I'm not a demigirl myself, but I think it's kind of not feeling a total connection to a gender, and that connection is not so strong to actually be that gender (I hope this makes sense?) Here's a short video about a demiboy who explains his identity: https://www.youtube.com/watch?v=CRKq9ouBr58
I'm not that familiar with the terms the OP is referring to, but I'd like to point out that transgender =/= gender dysphoria.
I use it as a technical title on this website ... I wouldn't expect anyone to take it seriously IRL because it is far from common parlance. If you wanna ask me what my experience is like, it's mostly a lot of wishing I looked and sounded female, while being apathetic about my current state of being perceived as a boy. (Reason being that if I start to care too much, it fucking sucks for me, and I'd rather reduce that creeping feeling of dissatisfaction even if it means forcing my mind to be distracted by other things.) So I guess it's hardly different from some other experiences of transgender girls. baconpox was actually pretty close to describing how I use the title. Because I subscribe to a worldview of relativism rather than absolute truths, I will never believe I have "figured myself out" all the way, for better or for worse. And I'd really love to feel indifferent about my gender, the greatest thing for me would've been being born as a girl who gives few shits about living up to a gender performance. That way people could still recognize me as female without me having to make extra efforts.