I'll try to answer base on what I know. Note I might be wrong though 😅

Just check the update page and make sure to install all app updates. If it shows no update then you're most likely in the latest version. If you're on stable channel, I believe push notification is enabled so you'll get one if there's a new update.

If you mean the link below, then no they're unrelated. That's just a job that builds the same image (or stable) over and over again and it was daily before so the number got really big. Just ignore these builds. You'll only need it if you want to reflash.
https://ci.ubports.com/job/rootfs/job/rootfs-pinephone-systemimage/

No

I'm not in stable channel on my pinephone right now so I can't see it myself and refer but in the OS build details page, you'll see different values such as UBports Image part and Device image part. OS Build number shows your current build number. Not sure if the pinephone shows "OTA" label or just build number for now. UBports image part is the build number/date for the rootfs or the actual OS. Device image part is the build number/date of the device specific software such as kernel. I'm not completely sure on this but this is what I know 🙂