If you're on PrEP, then you are required in most cases to be tested for not only HIV but other STIs every three months.
If you are on PrEP and properly following the regimen, the research shows that it is virtually impossible to be infected (greater than 99% effectiveness). Out of the millions on a PrEP regimen for many years, only a bare handful of infections have been seen, and those have very unique and rare circumstances associated with them.
If you are HIV+ but properly following the regimen and confirmed to be undetectable, then similarly you are virtually incapable of transmitting it. Transmission requires the viral load be quite large (studies show you need to be looking at a density of millions of virus particles in a sample, not one or two) to be infectious and if you're undetectable there's no virus there to be transmitted, or it's in such low numbers that you couldn't infect anyone.
In many professional porn scenarios, the producers often require recent STI and HIV tests before shoots. For more amateur situations, obviously there's no telling what they're doing. But again lots of them may well be on PrEP and getting tested with some frequency.
Obviously people on PrEP are warned that they are susceptible to other STIs and to use protection. But also most of those STIs are easily treatable and/or effectively inconsequential (such as most forms of herpes), so people make a risk/reward calculus and take the chance.
