How do I make a photo collage?

There are a couple of ways:

Use an app designed to do the job. There are several. Search on 'collage' in the App Store

Use any page-layout app - which includes anything from Adobe's INDesign down to Pages - and place the images on the page as you will.

By way of an example, here are some photos I simply dragged to the Pages app, and resized, added border and positioned in about a minute. Obviously a little more time needed to make it how you want it. For example to rotate the images slightly, or overlap them more etc.

Any Memory album will be presented by Photos as a tiled collage. It is a pity, that we cannot use the result directly to print this collage, as Photos is spending a lot of processing time on creating the Memories. But you could save the album you want to turn into a collage as an album und save it as a memory and take a screenshot, then use it as a template to help you arrange the photos in another application.

I like to create collages in Keynote, when I want to combine only a few photos.

Try can make any sized collage in Pages like this 20 x 30 size I did: