Choosing a healthcare provider

Hi all. Please forgive my ignorance on this because I’m new to the US and to Houston.

Recently I was googling covid testing providers for the purposes of some upcoming travel and I kept coming across comments on Reddit like “I’m with Methodist / MD Anderson / memorial Herman so I was scheduled in and got my PCR test results next day and it was $xx”

This got me thinking about health more generally, because I don’t want to have to work this stuff out when it’s a real emergency. I’m a healthy 35 year old, but healthcare in this country scares the crap out of me. My work provides insurance with BCBS, so I’m in a fortunate position but I’m still completely lost as to how to navigate the healthcare landscape in Houston. So I thought I’d turn to Reddit for a few questions:

1. What does it mean to be “with” one of these care providers? What kind of benefits does it provide? Is it necessary / desirable to do this?

2. How would I go about selecting which one of these is right for me? How would I join and how much would it cost?

3. If I’m not with one of these guys, how would I find a doctor or medical care if I needed it? Most of my medical needs are simple things like needing a prescription or something like that. But what if I had a more serious injury?

Thanks in advance.