You just have to let her do what she wants to do, dress how she wants to dress, and be who she wants to be. If she is trans, fine. But most likely, she's not.
This time has been horrendous for everyone, and I can't even imagine what it feels like to be a young person in the midst of all of this. I actually said to my partner yesterday, if lockdown had happened when I was a teenager who loved going out, seeing friends and even going to school, I don't know how I would have coped. I mean, I'm coping badly anyway, but my social life now is nothing compared to what I would have been missing out on as a teen.
With regards to the clothes though, there's no reason at all she can't wear whatever fashions she wants at a size 16. The majority of shops (save some very high end designer ones) cater for this size. You can almost certainly buy whatever she was wearing before in this size .. but that's not really the point.
The friendship stuff sounds like normal, teenage stuff .. but that doesn't make it any easier at the time of course. Encourage her to chat to the people she would sit with at lunch, or even join a club (there are youth groups running again now) to make new friends. A hobby could be just what she needs. Maybe a sports club or something?
She needs to be encouraged to live a little a differently, and not just sit at home stuffing her face (and trust me, I am an absolute hypocrite for saying this, as that pretty much sums up my lockdown, too).
Encourage her to go out for walks. It's not healthy for her to be sitting indoors and living a lonely, nocturnal life, so put your foot down on that one. She gets up in morning and goes to bed at night. She eats with the rest of the family. She does her school work during daytime hours.
Honestly, lockdown has been a very strange time, but it seems odd to me that you're so concerned about her wellbeing, yet you're allowing her to live like this. Sort her schedule out first, and other things will start to come into place around it, I'm sure.