How to make a "you have a new memory" movie with my own choice of photos and videos? The iPhone has a facility whereby, when it feels like it (typically after a trip abroad), it tells me "you have a new memory" and gives me a short movie with photos and video clips of its choosing. This can be customised to some extent by adding and removing photos.

Is there a way to select a bunch of photos and video clips and then invoke that movie-making engine on it?

Plus points if I can add photos not taken on the iPhone (e.g. on my DSLR). Extra plus points if I don't have to stick any of the photos involved in iCloud. Extra plus points if I can do the job on my mac rather than on the iPhone.
First, create an album from the photos you wish to include in your memory, or find an appropriate collection you wish to create a memory from, such as a moment.

On iOS, tap the collection header to show as a memory, then , then choose Add to Memories.

On Mac, open the album or click on the header to a moment, then choose Show as Memory, and finally Add to Memories.

You can include any photos from across your library taken with any camera and including video clips. You're not required to use iCloud Photos for this functionality since all functionality of this nature is powered by on-device processing.