Sideloading

Installing applications on a device outside the "approved manner" to bypass some restriction, like a modern Sneakernet

"Sneakernet is a slang term that refers to the transfer of data and electronic files between computers through removable media (like hard drives, flash drives and optical disks), which are physically transported between computers and on foot. This method is used in lieu of a computer network file transfer. "
—Technonet

I'm a firm believer in the freedom to use my device as I see fit, free from interference from Nanny. That means I get to use the software I need, as I wish to use it. Most vendors (Apple, Google/Android) want to control how I use my device through their software stores, so I am forced to take action to get the software (frequently open source) I want or need. At its core, sideloading means avoiding the "official" method of installing something (usually) software onto a device. It uses a form of the sneakernet, using a local, physical device (these days likely a USB storage device). The need is often driven by the limitations of the vendor's official "app store", and is both necessary (in the event that a particular application is not officially available in the "store") and desirable (if the official application image is hobbled or overly expensive). Seen by many as a necessary evil by those seeking freedom to use their device as they will, it's associated with the kind of hacker mentality. For me as a proponent of software freedom, it's part of my right to use the device that I paid for, as I need and wish to use it without being limited by the walled gardens set up by (say) the device or OS vendor.

One simply obtains a copy of an installation file (like an APK for Android) from a trusted source and manually install it on your device. Just make sure to tweak your security settings to allow installations from unknown sources. It's a great way to explore more apps, but always be cautious and download only from reputable sites to avoid any security risks. I say "simply", because finding a reliable source of that installation file can be tricky.

It's because of that latter issue that there exist many unofficial stores dedicated to sideloading applications, in both the Apple and Android stores. Obviously I have not tested all of them (and especially none of the Apple ones!), but I can confirm that F-Droid has a number of reliable apps that I continue to use. Aptoide seems to be the weapon of choice for many freedom-loving Apple users, with AppValley also getting a mention in my admittedly narrow research population. In the Android space there's also SLideme and GetJar (which I tested briefly) and I did find other recommendations out there which even a cursory search will turn up. If you're just starting out, there's currently a "top ten" list at https://www.mobileappdaily.com/knowledge-hub/app-stores-list, but the usual caveats apply (you may find something from a more reliable source!) I strongly advise you to get reliable reviews of any such store before using it, as there is a non-zero chance of getting inferior or badly-configured software at best and dangerous malware at worst (though I've personally never experienced the latter).

I've found it necessary from time to time to sideload media onto my phone . This is more easily accomplished these days, with the proliferation of USB devices and cables it's straightforward enough to copy files from my laptop to my phine (the hardest part can be finding the correct directory on the device, and some advance reading and research may be needed. Even Bluetooth can be used provided it stays up reliably for you (a chicken sacrifice may be required, of course).

This is one of those arenas in which I find myself siding with Richard Stallman and the Gnu Project's Four Essential Freedoms.

The freedom to run the program as you wish, for any purpose (freedom 0).
The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
The freedom to redistribute copies so you can help your neighbor (freedom 2).
The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.

Increasingly we see desktop operating systems (e.g. MS WIndows, Ubuntu) limiting access to their "stores". Ubunu are doing this via SNaps rather than allowing the user to install via the traditional, more open, software repositories. I've heard that if you try and bypass the Ubuntu Snap store by using apt it installs the Snap anyway. Freedom to use one's computing device as one would wish is part of a philosophical and political stuggle, and sideloading is one of the tools used to ensure and maintain that freedom. There are many who would deny us these freedoms for their own purposes, purposes that are clearly not taking into account our rights and needs. Rise up, sideloaders and take back what is rightfully yours. Viva la Revolución!

$ xclip -o | wc -w
719