How does iCloud sync work on iPhones in regard to photos?

I have an iPhone and a new MacBook. Both have iCloud photo sync turned on. I lost my phone while on a trip. While I was on the trip I never actively did anything to "update" or "sync" my phone to the iCloud. I was kind of hoping that with iCloud photo sync turned on, as I took photos, it would automatically update to the cloud, eventually. When I finally got home (sans iPhone), hundreds of miles away from the last known location of the iPhone, I hopped on to my laptop in the hopes that it would have the photos synced up and in the iCloud, however only about 10 photos saved from the whole trip. And very randomly. I clicked on "sync" and connected my Mac to power, but I'm assuming the phone also needs to be connected to power as well?? And do they need to be close by? I called the phone a little while ago and it still rings, as though it is still on (this occurred about 24 hours ago). Does it need to be "on" to sync what's on there to the cloud? I'm having a panic attack thinking about the incredible photos I've lost. I'm assuming if they didn't already sync by now, they're not going to, but I figured I'd ask. I don't really understand why some photos sync and some don't, it seems a little erratic which ones do and don't. Thanks for the insight!

[Re-Titled by Moderator]