How to merge free space into the previous partition?

To be clear, you have two actual APFS particions on a single HDD or SSD. You're deleting one, and you now want the remaining partition to consume the entire device, right? Because if you have a single partition with multiple volumes, you'd simply delete one of the volumes and not worry about it.

Here's Apple's documentation. Go to the subheading for enlarging a partition. I hope it helps.

I didn't actually read it, but it seems like you'd select the existing partition, go to the Partitions tab in the Disk Utility toolbar, and enlarge it from there. I understand they have some kind of slider control to change the partition size, and I'm sure it's quite glitzy and really floors people who have never used a Mac before. But I assume there's an accessible alternative, such as a text field that lets you type a size.

Sorry, it's been a long time since I did anything like this, and I really don't want to go anywhere near this interface with my existing partitions selected.

If it is an APFS volume, the space is reclaimed automatically and becomes part of the APFS container as soon as you delete it.

If it is a physical slice (partition), deleting it will leave behind free space that, if the partition to its left can be expanded to fill it, Disk Utility's Partition tool will automagically do for you by default when you apply the changes. Do check that the configuration is right before you do it, obviously. The process is accessible.

Otherwise you will have to do more intervention than this and I advise caution. Well, frankly, I advise caution anyway, because although this sort of hot partitioning is extremely sexy, it can and does sometimes go wrong. Keep backups!