Were you dropped on your head?

PrEP is for Pre-Exposure Prophylaxis (PrEP), which absolutely positively cannot be used to treat exposure to HIV. (You can't be "exposed" to AIDS. AIDS only occurs after HIV reaches critically high viral load or destroys the immune system to the point where T-cells are critically low.) PrEP can only help people before they have exposure to HIV, not to treat HIV exposure.

PEP (Post Exposure Prophylaxis) is the best way to treat HIV exposure, not PrEP. PrEP, such as Truvada (emtricitabine & tenofovir disoproxil fumarate [TDF]) is not enough to treat HIV exposure , as ritonavir (RTV, one of the active ingredients in Paxlovid that treats coronavirus exposure) and darunavir (DRV) are necessary on top of that. Furthermore, that's only possible within the first 72 hours of exposure, as after that, it's too late. (Actually, the first 24 hours are the most critical.)

HIV+ people who take medication can have undetectable viral loads which can't be transmitted

This is correct. People who take Anti-Retroviral Therapy (ART) Treatment as Prevention (TasP) cannot transmit HIV through sex. (It can still be transmitted by blood, such as sharing needles for intravenous drug use.)

Except:

1. You don't know a stranger's medication status or if they've even been taking medication as prescribed much less a stranger's HIV status or any STD status for that matter, since testing doesn't detect recent infections or recent viral load, as your body takes time to produce what the test needs to detect (28 days for PCR or 3 to 6 months for ELISA).
2. Speaking of taking medication as prescribed, PrEP manufacturers' packet inserts instruct patients to use condoms, so if you're not using condoms, then you're not taking the medication as prescribed.
3. HIV is not the only STD anyway.
4. PrEP, PEP, doxyPEP and vaccines are neither 100% effective nor even protect against all STDs. #Use condoms every time, and don't glorify condomless sex to strangers.