There's a book called Tabletop Role-playing Therapy: A Guide for the Clinician Game Master by Dr Megan A. Connel that's a really standout resource about this, she appeared on the official D&D podcast a year or so ago talking about it.

I'd say that this is more a resource for therapists to use TTRPGs than it is for DMs to act as therapists for their players. There's a fine line between accommodating your players' preferences and needs and providing unwanted therapy; if you want to actually put any therapy techniques into your game, ask your players approval first.

Every work of artistic expression, from painting, through photography, all the way to fantasy improv, is a little window into who you are

the imaginary you is being put in various situations, mysteries, plots, and they have to figure out their way to victory! (and at the same time reveal exactly how you appraoch problem solving, how your logic works, and what assumptions you make)

it's like the random shapes test but you don't even realise you're doing it

though it could get significantly harder to see through the layers of fantasy if you're dealing with experienced roleplayers playing their 50th character