I’m 99% sure they ripped the note editing functionality from Standard Notes.
Which would upset me, except Standard Notes borrowed a lot of code for the editors from other people, and try to sell them back to you with a yearly license, even if you host your own server.
“Rip” is probably too crass a word. But there’s a lot going on with fancy text editors written in JavaScript, just below the surface. They are much harder to build than you might expect!
Many of Standard Notes’ editors are wrappers for open-source text editors that were created by somebody else*. Today, Standard Notes’ premiere note type is the Super Note. It is also a wrapper around Facebook’s Lexical Editor. (Since Lexical is MIT-licensed, SN can legally use it and charge people money.)
When I look at Filen’s Note UI, it bears a striking resemblance to Standard Notes’ advanced editors. It’s also implemented in HTML on both mobile devices and its web interface. I’m reasonably certain they did not reinvent the wheel, and simply copied from Standard Notes in a way similar to how Standard Notes copied a bunch of third party plugins.
* Standard Notes charges you recurring fees for the privilege of using Super Notes, even if you shoulder the burden of self-hosting it. There is nothing unethical about reusing code from an open source project, but I have disdain for these fees, specially when SN could/should simply offer paid plans with the promise of convenience. This is how BitWarden and Ente make money without being hostile to self-hosted users: offering convenience as an option.