What is the best way to make a collage on Photos with multiple photos, please?

If you do not want to purchase an additional application, you can use Keynote to arrange several photos on a page or Pages.

You can download Keynote and Pages for free from the App Store, if you have not already done so.

Get an app.

Example: https://apps.apple.com/app/pixelmator/id407963104?mt=12&ign-mpt=uo%3D4

There are others, search Mac App Store.