It's still severely locked down Android. You can enable some hand-holding kid gloves "developer mode" features that go ever so slightly above what a normal user would be able to do but be prepared to sacrifice your firstborn just to get developer access (requires having a phone paired to the device with your personal Facebook account so no chance of sharing developer mode features with anyone else).
Trying to do file management on the Quest platform is a nightmware. With a whole lot of work you can flash some Android apps that can do things like SFTP but it's impossible to keep the headset "awake" long enough to complete large file transfers. Not only that but it'll auto kill such apps that have been open for a while without activity.
You have no mechanism to schedule tasks or execute arbitrary scripts. There's no command line access to speak of. Closest you can get is locked down adb.
Apps can access other apps storage areas if they've designated that area as shareable (e.g. not part of the app itself). So that means if you say, mod Beat Saber (which is very complicated/difficult compared to PC) the downloaded maps can be accessed by a normal Android file sharing application. But like I said, the headset will turn itself off and occasionally just plain kill any app that's running without activity (maybe due to lots of network activity? Dunno).
Trying to keep the files on your headset in sync with anything anywhere is always a manual process. There's no way to script or automate anything. You also can't install URL handlers that would say, open another app to handle a particular filetype (that's locked down). I haven't tried it but I also haven't seen a mechanism to install your own certificates.
There's also no way to set the clock (LOL). Once you pick your timezone when you first power on the device (and mandatory pair it with your personal phone/Facebook account) you must use that timezone forever. This means that if something goes wrong with the date (as did happen with my headset) the device will no longer accept any SSL certificates which means no updates and basically everything stops working on the headset. The only fix for this is a factory reset.
Meta support is also total garbage. If you have an issue and it's not part of their script of answers or something they've encountered a million times it's not getting fixed. FACTORY RESET is what they say over and over again because Meta wants the headset to be perceived as an appliance and most certainly not as a general-purpose computing platform.
That last fact is what I think holds back the entire platform as a whole. If they used a regular Linux distribution and gave developers the power to use it like a normal Linux distribution it would accelerate innovation and vastly increase the usability, availability of apps, and overall improve the user experience.