Yes, it's all drives, even HDDs.
Unless you convince the installer otherwise - the Mojave installer changes the format of the destination partition to APFS, even if there are other non-APFS partitions on the drive. I installed Mojave on an external HDD (spinner), where I have several older macOS systems installed. There's a total of ten partitions on that drive. The Mojave partition is now APFS. The other partitions remain as HFS+, all on the same hard drive. All continue to boot just fine on Macs that support them. Oldest is Leopard. Latest is now Mojave.