protocol for clinical trials with psychedelics wrt depression, PTSD and OCD

[insultium]

while I was watching some documentaries lately I was wondering what sort of preparation people went through that had no experience with psychedelics before they were given psilocybin or LSD in a clinical settings with regards to depression, PTSD and OCD.
I do have a thorough history with psychedelics but still would hate to have several sober people together with me in the room that monitor my every move and take notes.

My primary questions are these:
How do they psychologists prepare the individuals?
When do they interact or do they interact at all?
How large a dose is given to them?
Why are most trials not in the outside nature but in a closed environment, sometimes I saw them with a blindfold on.

found this along with my questions. Did not check erowid and bluelight, in case someone has an idea where I can find infos on my questions, thank you.
https://clinicaltrials.gov/ct2/show/record/NCT03866252?term=LSD&cond=Major+Depressive+Disorder&draw=2&rank=1