FrostWire (https://github.com/frostwire/frostwire) is also open source, not as popular as LibreTorrent though it does have other features built in (torrent search for example).
Both are also in the Android Play Store though you may want to install the .apk directly if you need the app to have full access to your Android storage.
LibreTorrent (https://gitlab.com/proninyaroslav/libretorrent) is popular.
FrostWire (https://github.com/frostwire/frostwire) is also open source, not as popular as LibreTorrent though it does have other features built in (torrent search for example).
Both are also in the Android Play Store though you may want to install the .apk directly if you need the app to have full access to your Android storage.
Seconding LibreTorrent, works great for all my mobile torrenting needs