The website that BlueSharkEnjoyer shared is a really great place to get information about DIY transitioning. Also, r/transDIY if you still have access to Reddit.
I'll just comment on your third question, because there are various reasons why someone might want to go the DIY route. Some quick examples I can think of are:
Laws that make it hard to access HRT through traditional means.
Long wait-list times to have access to gender-affirming treatment.
Gatekeeping practices that make it unreasonably hard to access HRT.
DIY can be lower cost, especially for people who are uninsured or who don't feel the need to see a therapist.
People who are insured with their parents might not want them to see their treatment on insurance statements. (My reason for doing DIY HRT)
Some people are just "testing the waters" and don't want a trans diagnosis on their medical record.
Some people might not consider themselves to be trans, but want to take HRT anyway with minimal hassle.
More control over dosage and medications.
Ability to stockpile medication in case it can't be obtained in the usual way.
I'm sure there are other reasons. Those are just the ones I can think of at the moment.