Finder: Sync photos to your device from ....

I'm in the process of trying to organise years worth of photos. The vast majority of photos are stored on the hard drive of my Mac (within multiples folders as transferred from a PC) with more recent photos held in Photos on an iPhone. I'm researching the different options of how to consolidate so that all photos are in the same place, in the same format etc but first need to feel confident I understand how photos are actually managed by Apple.

I understand iCloud Photos is probably the best option, but for now I'm trying to understand how syncing via Finder works.

One thing I can't find any information about is the two options for syncing photos via Finder. So I can connect my iPhone to my Mac and using Finder, select Photos from the menu and check the 'Sync photos to your device from:' box. I then have two options for sources:-

1) Pictures (and then to select folders)

2) Photos (and then to select albums)

I understand the difference between the two options, but what I want to know is, can I do both without one overwriting the other?

So, can I sync photos to my device from Pictures (option 1 above), and once that has been completed can I then sync photos to my device from Photos (option 2 above), so that my device now holds photos from both sources?

Or will the second sync overwrite the first so that my device only contains photos from that second sync / second source?