If you've been watching the news cycle around app bans and the policies around what is and isn't allowed in the app stores run by Apple and Google, you might have also seen references to "sideloading" apps—which essentially means installing apps that aren't available in the Google Play Store or the Apple App Store.
This is actually relatively easy to do, at least on Android. It can give you access to apps you wouldn't otherwise be able to run (usually because they violate one or more app store policies), and it can also enable you to try out early versions of apps before they're released to the masses.
Before you rush to start downloading and installing things, it's important you know that sideloading apps removes the security protections that Google and Apple give you: Their app stores are carefully patrolled for malware, stalkerware, and anything that might try to steal your data or break your device.
By installing unauthorized apps from alternative sources, you're sidestepping these safeguards. Be very, very careful about which apps you choose to sideload—check and double-check the background of these apps and the backgrounds of the developers that make them. In general, you should install apps outside of your phone's app store only if you have a very good reason to. And if you just read that thinking "Oh, I'll be fine, I don't need to do that extra research," you probably won't, and you really really shouldn't install it.
How to Sideload Apps on Android
Sideloading apps on Android isn't particularly difficult, as long as you're cautious about what you install and take into account the security considerations we've mentioned above. That said, it can be a useful way of expanding what your phone is capable of. It also lets you play around with apps outside of the Google Play Store.
To get started, point your mobile browser toward a trusted site that stores APKs (Android Packages). APKMirror is one of the best and most reliable, for example. When you've found the APK that you want to install on your phone, tap the Download APK button and then OK to confirm you understand what Android tells you—that unauthorized apps can harm your device. At the risk of sounding like a broken record, make sure you know what you're doing.