When signing into iCloud with a new iPhone, what happens to recently taken photos/videos already on the device?

[bwApple! fwApple!]

So suppose I get a new iPhone 15 right before I go abroad on holiday for a few weeks.
My old iPhone that I've used for everything over the years is a 2016 SE, and I take it with me, using it for everything I've always used it for.
I'd prefer to transfer everything on that SE to my brand new iPhone 15 after I'm back home. (After a successful transfer, the SE is to be wiped and to disappear in a drawer of old electronics.)
But, while on holiday, I'm already using the iPhone 15 for shooting a whole bunch of photos and videos because after all the 2023 iPhone 15 camera is so much better than the 2015 (6s) camera that is in the 2016 SE. (So, for a few weeks, it will function only as a camera.)

When I get back, and I sign in to iCloud, for the purpose of transferring everything on the SE -or, everything in iCloud, as everything on the SE is synced to iCloud- to the 15, will there be a merger of the library of newly shot photos and videos on the 15, and the old library (with hundreds of gigs worth) of photos and videos on the SE?

I would prefer not to run the risk of: finding that my supposed "new library" on the 15 "overwrites" the old photos library on the SE/iCloud; or, finding that the "new library" on the iPhone 15 gets "overwritten" by my old SE/iCloud library.

I hope that made sense.

Thanks!

 

[FreakinEurekan]

What will happen? ¯\_(ツ)_/¯ it you do everything right, all will be merged and you go on with your life..

But that’s a huge “if.”

I’d recommend transferring before you leave.