For what it’s worth, it’s probably worth the time to discuss what subset of what in Apple’s tech stack supports what.
macOS and iOS: different everything, but with a high degree of commonality. The kernel is closely related (possibly even identifiable as largely the same thing, like Linux and MCLinux), and the userland definitely maintains some degree of relationship. This likely holds true even on Apple Silicon based Macs. macOS could be viewed as cousins, perhaps.
iOS and iPadOS: the closest relatives among Apple’s operating system efforts. They have a lot in common, but the iPad software isn’t “iOS plus iPad features”, the iPad quite often is lacking in iOS features in addition to having its own set of features. They could be viewed as fraternal twins, possibly as identical twins with different physical characteristics (like weight and hairstyle or fashion sense) [but I hesitate to call them identical twins because that could imply having the same code through and through, though they are more closely related than just siblings]. iPadOS does tend to tack close to iOS, but it’s more than possible for them to support different hardware if the code restricting hardware support is outside of the common subset of software they have in common.
iOS and tvOS: have similar kernels (with tvOS seemingly having forked the iOS kernel at some point, likely to maintain support for older A series chips). Userlands are quite similar but tvOS definitely has its own things that don’t have any analog to things in iOS. Could be considered siblings.
iOS and watchOS: watchOS is certainly derived from iOS, its kernel and userland are probably forks from iOS’s and the hardware it supports is dramatically different (static RAM over DRAM, no high performance CPU cores [both CPU cores are high efficiency], that sort of thing). (Though Wikipedia notes that the HomePod mini uses an S5 SoC, and the HomePod/HomePod mini uses a version of tvOS, so the kernels may not be that different.) Siblings, maybe half siblings.
TL;DR: Basically all of Apple’s current operating systems could justifiably be considered different things, even if they’ve got more in common than, say, Windows CE and Windows NT. This even extends to iOS and iPadOS, even if they have the most in common out of all five. There’s a lot more to them than just the kernel and the common set of code shared by both userlands.