"photos and videos synced from Finder will be removed"

On Windows PCs, and on Macs running Mojave or earlier, you use iTunes. (Note: You wouldn't use any of the Mac installers for Mojave. You'd use the iTunes the Mojave installer gave you.)

iTunes - Apple

Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

Macs using Catalina or later can't run iTunes. Apple scattered most of its functions between the Finder, and new applications called Music, TV, and Podcasts.

What happened to iTunes? - Apple Support

Use the Finder to sync your iPhone, iPad, or iPod touch with your Mac - Apple Support

Lanulos Said:

"'photos and videos synced from Finder will be removed': how can I tell what they are'"

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Turn On Finder Syncing, First

You need to turn on Finder syncing. Then, all will be set. This would be performed on your Mac.

1. Go here: Sync your Photos Manually Using the Finder - Apple Support
2. As it Reads: "turn off syncing in the Finder and they'll be removed from your device. You can turn off syncing for some of your photo albums or all of them at once."