One of the most popular ways is to use the F-Droid repositories, which if you know a little Linux concepts it's like plugging in another software repository to the same package manager. (see f-droid.org) It can be confusing though because F-Droid is both an app, and the name of the primary software repo which is pure FOSS software (no ads, no trackers).
The F-Droid app supports adding more repositories (think like apt/yum/dnf on Linux) easily, so you can source software from anywhere which runs their own repository. One of the most popular "other" repos is Izzy (apt.izzysoft.de/fdroid), and there is an alternate project called "microG" which can allow you to use Google Play store apps (microg.org/fdroid.html). microG is how you will get your Google Play apps onto the device, usually (there are other solutions besides microG out there however).
The CalyxOS install ROM includes F-Droid (app and repos) and offers to install microG for you on your first boot (as well as some other opt-in stuff). Calyx runs their own F-Droid repo which is pre-added to the app so you get updates from them as well (think the built-in apps most smartphones have).
Nit rebuttal: I was referring to the F-Droid repository which I thought was clear from context. These elements are scanned for and apps called out (tagged) should they contain something not-free, even connecting to network services like Reddit or Twitter. The are referred to as Anti-Features: https://f-droid.org/en/docs/Anti-Features/
Trivia: by default (unless it has changed upstream), the F-Droid app defaults to "Include anti-feature apps: Off" in the Settings. The user must go in there and manually opt-in to see all the anti-feature apps on the mobile client.
It doesn't actually hide apps with "anti-features", you can still see them by default. The only thing it does is hide the description and install button of apps with "anti-features" in the search screen. It seems like a half-baked feature.
That's true, but the date of the most recent release is clearly shown, and it's easy to avoid the unmaintained ones. Also, F-Droid most likely has newer alternatives for the kind of app you're looking for.
The F-Droid app supports adding more repositories (think like apt/yum/dnf on Linux) easily, so you can source software from anywhere which runs their own repository. One of the most popular "other" repos is Izzy (apt.izzysoft.de/fdroid), and there is an alternate project called "microG" which can allow you to use Google Play store apps (microg.org/fdroid.html). microG is how you will get your Google Play apps onto the device, usually (there are other solutions besides microG out there however).
The CalyxOS install ROM includes F-Droid (app and repos) and offers to install microG for you on your first boot (as well as some other opt-in stuff). Calyx runs their own F-Droid repo which is pre-added to the app so you get updates from them as well (think the built-in apps most smartphones have).