I’m creating a pool on a USB connected SSD using space left over from an RpiOS install. The RpiOS uses MBR partitioning (and I have no choice about that AFAIK.) In creating the partition which I’ll format as ZFS, I can choose between an extended or primary partition. Does this choice matter in any way to ZFS?
Nope, it doesn’t care in the slightest.
As a matter of fact, when you feed ZFS a “full disk” what it actually does is immediately partition it into part1, part5, and part9. Part1 is the only primary partition, and it’s only used as a placeholder–your actual data goes into part5, which is of necessity an extended partition.
On the other hand, my personal most common deployment is a UEFI boot partition on part1, a / on part2, and ZFS on part3–no extended partitioning whatsoever. So, like I said: ZFS truly just does not care.