Linux

As an Arch user (btw), that's rarely an issue thanks to the AUR and it's vast package pool :) But on the very rare occasion that it's not there on the AUR but available as a deb, I can use a tool called Debtap to convert the .deb to the Arch's .tar.zst package.

For rpm-based distros like Fedora and OpenSUSE etc, there's a similar tool called alien that can convert the .deb to .rpm.

In both instances though, dependencies can be a pain, sometimes you may need to trawl thru the dependencies and convert/install them, before to do the main package.

Ideally though, you'd just compile from source. It used to be a daunting task in the old days, but with modern CPUs and build systems like meson, it's not really a big deal these days. You'd just follow the build instructions on the package's git repo, and usually it's just a simple copy-paste job.

Finally, if these packages are just regular apps (and not system-level packages like themes etc), then there are multiple options these days such as using containers like Distrobox, or installing a Flatpak/Appimage version of that app, if available.