My understanding is that on iOS your only non-paid choice is iCloud, and iCloud doesn’t reliably sync to non-Apple systems. To clarify, the use case here is that you have an iPhone but also non-Apple systems, which is a fairly common scenario.
TBH that sounds like a better reason to get something other than Apple systems instead of changing all your apps to work around Apples bizarre limitations.
FWIW there is iCloud on Windows [0]. There seems to be some Linux options, but nothing official, so it seems like it'd be a lot more effort there. Also no official Android app it seems.
