My old Mac: late 2013 MBP with 512 GB. The Photos library wouldn’t fit on the internal SSD, so it’s been on an external SSD.
My new Mac: Brand new 2021 MBP M1 Pro with 2 TB internal SSD. After migration I moved the external Photos library to my internal drive.
I have 2 TB of iCloud storage and have been syncing my Photos all along, so they are in iCloud.
On my old Mac I changed my iCloud settings to “optimize disk storage.” That’s the only change I made.
If I open Photos it says it cannot find the Photos library. I tried finding it in Pictures, the new location on my new MBP, but it doesn’t seem to be there.