My read of it is they're basically like independent contractors, and Thriveworks is just a service to better connect clients with therapists. My therapist and psychiatrist haven't once mentioned Thriveworks in about a year I've been seeing them (anxiety, death in the family).

I would still suggest maybe checking in with your PCP first. That's where I went to first, and my PCP gave me various recommendations for mental health. Could be they have someone they trust they could recommend to you. Good luck!!