The principled "old" way of adding fancy features to your filesystem was through block-level technologies, like LVM and LUKS. Both of those are filesystem-agnostic, meaning you can use them with any filesystem. They just act as block devices, and you can put any filesystem on top of them.

You want to be able to dynamically grow and shrink partitions without moving them around? LVM has you covered! You want to do RAID? mdadm has you covered! You want to do encryption? LUKS has you covered? You want snapshotting? Uh, well...technically LVM can do that...it's kind of awkward to manage, though.

Anyway, the point is, all of them can be mixed and matched in any configuration you want. You want a RAID6 where one device is encrypted split up into an ext4 and two XFS partitions where one of the XFS partitions is in RAID10 with another drive for some stupid reason? Do it up, man. Nothing stopping you.

For some reason (I'm actually not sure of the reason), this stagnated. Red Hat's Strata project has tried to continue pushing in this direction, kind of, but in general, I guess developers just didn't find this kind of work that sexy. I mentioned LVM can do snapshotting "kind of awkward"ly. Nobody's done it in as sexy and easy way to do as the cool new COWs.

So, ZFS was an absolute bombshell when it landed in the mid 2000s. It did everything LVM did, but way way way better. It did everything mdadm did, but way way way better. It did everything XFS did, but way way way better. Okay it didn't do LUKS stuff (yet), but that was promised to be coming. It was Copy-On-Write and B-tree-everywhere. It did everything that (almost) every other block-level and filesystem previously made had ever done, but better. It was just...the best. And it shit all over that block-layer stuff.

But...well...it needed a lot of RAM, and it was licensed in a way such that Linux couldn't get it right away, and when it did get ZFS support, it wasn't like native in-the-kernel kind of stuff that people were used to.

But it was so good that it inspired other people to copy it. They looked at ZFS and said "hey why don't we throw away all this block-level layered stuff? Why don't we just do every possible thing in one filesystem?".

And so BtrFS was born. (I don't know why it's pronounced "butter" either).

And now we have bcachefs, too.

What's the difference between them all? Honestly mostly licensing, developer energy, and maturity. ZFS has been around for ages and is the most mature. bcachefs is brand spanking new. BtrFS is in the middle. Technically speaking, all of them either do each other's features or have each other's features on their TODO list. LUKS in particular is still very commonly used because encryption is still missing in most (all?) of them, but will be done eventually.