Linux

To be honest, VMs are probably the easiest way to do it, like many said already. You don't need to deep dive into virtualization to set up a few VMs and use them to learn about different distros. No need to think about how to partition your drive, mounting swap partition, disable hibernation... Want to try out another OS? Just create a new VM and you a ready to use it. You can simply create shared folders between host and guest and enable shared clipboard, if you want. Switching between the different systems is much easier than shutting down and rebooting another OS. You can even run them at the same time if you want and your hardware is good enough. And aside from the better convenience, the potential to break something is much less compared to tinkering around with a multi boot system, imo.

Just pick a distro. It sounds like you want to learn. I suggest arch. It does the least for you, is the least opinionated, but also has by far the best documentation (arch wiki is the de facto linux documentation).

The difference between the distros is otherwise simply what package management tool they use, and what packages are in their repository. Nothing else is different that's of any importance.