Where to put Lightroom Catalogues in new APFS system

I kept a separate device with my Adobe Lightroom catalogues and other Photography work files

Do you mean that you used a dedicated HDD (internal or external) for these catalogues & files? If so, have your reasons for doing that changed? I prefer to keep my data completely separate from my system drive, so for data I either create a second partition on the system drive and/or use a second drive exclusively for data.

In a default APFS installation the entire system drive is formatted as APFS and all space is shared dynamically. If your system is all SSD this is perfectly fine from a performance standpoint. But if your system is HDD based this can end up giving you performance problems. Mike Bombich wrote an informative article on the differences between APFS and HFS+ performance on HDDs (rotational media) that you may find interesting & revealing. There is also a post at OWC about APFS on HDDs. (Even though the articles are about HDDs they are good reads about how APFS manages files overall.)

Because I do video as well as image editing, and my Macs are still HDD based, on my newest iMac I partitioned its single drive with a 250GB system partition as APFS and a second 750GB (data) partition as HFS+ in order to maximize performance especially when I am editing video. I had my reasons for sticking with HFS+ for the second partition vis-a-vis video editing - reference the two articles above.

What was the point of the partition before?

Why you wanted it organized like that probably affects how you do it now on APFS.

Under APFS, there are Containers that are much the same as a partition.

A container has Volumes in it which are almost virtual partitions. They are mountable divisions of the container, but they share the same storage space with all the other Volumes in the Container. When you create them, you can set size quotas on the Volumes, but you don't have to.

So, you could add a Volume to the Container or you could partition it which would add a new Container where you could virtually split it up with Volumes.

Partitioning the drive restricts the amount of space dedicated to that container whereas adding a volume doesn't.