[Google Pixel 8 Pro] CHH2:
There's about a hundred different ideas, perspectives, "rules", and "guidelines" out there.

The easiest question to answer in theory is knowing which lens you are using. The bar at the bottom of the frame lets you select the ultrawide (.5), wide (1x), and tele (5x) lenses with that weird 2x thrown in. I said in theory though because unless you tell the Pixel Camera app in the Pro Controls menu setting that you want manual lens selection instead of auto, the phone will just choose what it decides is best all while telling you, "Yeah, sure, we're on the 5x lens you chose in macro mode." In reality, despite saying it is using the 5x lens, it's really just cropping the .5x ultrawide which is the only lens that has macro mode. It does this A LOT for various cases. If you put it into manual selection, it'll give you exactly the lens you ask for but then you are bound by its constraints such as minimum focus distance just like you would on a standalone camera.

The rest of it gets down to coming up with a composition and trying to make it look as best as you can. Wide lenses open things up, space them out, lets things breath in the frame. Teles "zoom in", compress, and flatten. There's probably only a couple of rules/guidelines to really take into consideration.

One is the wider the lens, the more exaggerated any tilt in the camera is going to be. Architecture is a huge place for this rule. Big building, you're close up, so you put the bottom of the building in the bottom of the frame and tilt the camera back until the top fits in. The building is going to look like it is falling back from you because the camera is actually falling back from the building. Only two ways around this. Either shoot a wider lens and realize you're about lose A LOT of your available frame to cropping... or use a specialized camera/lens that features tilt/shift functionality.

Second, and this is the thing that is absolutely baffling about cellphone cameras, taking people's portraits with anything less than 40-50mm is not going to be flattering. Oddly, pretty much all "normal" lenses on cellphone cameras are far wider than flattering focal lengths. Ideally, the flattering range is in the 85-105 and maybe even 135 range. The teles on pixels are usually somewhere between 112-130, I think, depending on the year.

Outside of that, I'd just experiment with framing stuff up and seeing how it looks to you. Even if you can't exactly name off everything going on in the frame and why it does or does not look right, you probably have a feeling when something seems off or could be better. That's the great thing about photography these days, you get instant feedback. As long as it isn't something fleeting, you can take a photo, adjust, and reshoot. I'm a little jealous of people picking up and learning photography in the digital age.

And I guess the only other thing I'll say is just realize not every photo has to be prize winning fine art. My camera roll is full of "I need to remember this mundane life detail", "oh, hey, that looks cool but not perfect so I'll take it for the memory", "that looks good", "that looks great", "that looks absolutely amazing", to "yeah, we're going to take that and sit on it for a while". Cellphones are the handy camera. My standalone cameras are intentional.