I would take a guess that lots of people take PrEP but skip the STD testing...
It would also be dangerous to be taking PrEP and not getting the required kidney function tests with it ever 90 days (the older version of PrEP can be hard on some people's kidneys).
I do see advertisements for legitimate mail-order services where you do consult with a doctor, and perform the tests at home.
Looking at one site, if you have no coverage, it's $164 for kidney and STI testing, $94 for just the required kidney tests. So some may just opt out to save money. But again, I would be surprised if there's many people paying out of pocket for the whole treatment.
You can get the kidney function tests yourself if you like on demand, as well, privately.
I order my PrEP online and do all my STD testing at home, since I hate having to reveal my sexual history to strangers constantly.
Just want to add this here in case anybody's unfamiliar: PrEP can also be taken on an event-driven schedule, rather than the daily dose most people are familiar with. Under this regime, you take two doses two hours before sexual activity (or upto 24 hours earlier), followed by a single dose 24 hours after the first 2 pills, and a final dose 24 hours after that. If sexual activity can be planned or is infrequent, this 2+1+1 regime might be preferred.
As the WHO notes, the event-driven regime isn't generally recommended for adolescents who might have issues with compliance, or cis & trans women.