Some other FOSS apps I use daily:

Aegis - 2FA (https://github.com/beemdevelopment/Aegis)

Breezy Weather - A very good looking weather app (https://github.com/breezy-weather/breezy-weather)

OnlyOffice Documents - MS Office suite replacement (https://github.com/ONLYOFFICE/documents-app-android)

Fossify Calendar (https://github.com/FossifyOrg/Calendar)

Fossify Messages (https://github.com/FossifyOrg/Messages)

Aves - Local gallery with great organization (https://github.com/deckerst/aves)

Termux - Terminal emulator (https://github.com/termux/termux-app/)

Unexpected Keyboard - A unique keyboard that pairs nicely with Termux (https://github.com/Julow/Unexpected-Keyboard)

WG Tunnel - WireGuard client (https://github.com/wgtunnel/wgtunnel)

These are all easily installed through Obtainium: https://obtainium.imranr.dev/

Some others that I use:

* NextCloud -- client for personal NextCloud server; this app is used primarily for file sync, with other features accessed with other apps. (https://nextcloud.com/features/?filter=Clients#android-clien...)

* KeePassDX -- password manager, shares DB with KeePassXC on desktop, which is synced via NextCloud. Also functions as a TOTP authenticator. (https://www.keepassdx.com/)

* DAVx5 -- CalDAV and CardDAV client; keeps mobile calendar and contact list synced with private NextCloud server. (https://www.davx5.com/)

* AntennaPod -- excellent FOSS podcatcher. (https://antennapod.org/)

* KDE Connect -- desktop sync tool; allows file/clipboard/keyboard/audio/etc. sharing between phone and a Linux desktop. (https://kdeconnect.kde.org/)

* Kore -- remote control app for a Kodi instance running on your LAN. (https://kodi.wiki/view/Kore)

And I don't see F-Droid itself mentioned -- it's the most popular repository of FOSS software for Android, with an accompanying app: https://f-droid.org.