iCloud

1) If you have Desktop & Documents on iCloud Drive enabled on both, then it most likely will.

2) Only Desktop & Documents folders are synch'd with iCloud Drive (if you have it enabled). There is no way to enable any other location to auto-sync.

You can sync your entire Photos Library to iCloud, but there is no other way to automate that.

You can manually move files into iCloud Drive.

You said "You can sync your entire Photos Library to iCloud..."

First, you need to ensure you have enough iCloud Storage.

Second, you must be using Photos to manage your pictures.

Then, you can turn it on in Photos preferences or in Apple ID System Preferences.