How the heck does an HSA work

My entire life I have had PPO. I regularly hit my OOP max for the year. I started a new job this year and the only offering was an HSA. I am so confused by how it works that I’ve avoided my doctors, despite having some chronic conditions that require routine care.

I have an Aetna HSA and then I have pre tax funds go into an Optum Financial account. The few claims I’ve had so far are expensive- $103 for a 15 min check in with my primary, when my copay was $25, and $125 for a therapy session that used to be $40.

There are also things that Aetna won’t cover, like some prescriptions I was on. I feel like I can’t afford to talk to my doctor about what else I can take, so I’m not sure what to do when I run out. In one case I’ve stopped my medication altogether.

I guess I’m looking for some kind of guide to understanding how an Aetna HSA works and how the Optum money works. For example, if Aetna doesn’t cover a medication and it winds up being $330 OOP, can I use the Optum debit card for that? Or can I only use the Optum money for Aetna claims? And if the latter is true, how do I actually use the card? So far I’ve just paid for these visits myself but it’s realllly expensive. I also hate not knowing the cost of routine visits until weeks later but when I called my primary dr they told me they can’t give me a billing estimate in advance since it’ll depend on what the doctor codes my visit at. It’s all very overwhelming. I have an ultrasound scheduled next week, since it’s done in office this would be covered under my copay, but now I’m terrified I’ll get a $500 bill and not know til after the fact.