Mitigating photos from one Mac to a new Mac

If you have a time machine backup, you can simply plug it into the new Mac as you start it up and it will transfer everything automagically.

If your photos are on an external drive, simply plug it into the new mac - you might need a USB-C to USB-A adapter - and just use it as you did before.

Can you tell us a bit more about your situation and just where your photos are stored?

If you are using iCloud Photos on your old Mac, you can use iCloud to transfer any photos stored in the old system Photo Library (the one that synchronizes with iCloud) to the new Mac.

You would still need to make separate provisions for photos stored outside of that library.