You can set a different bass note for a particular chord. If you do so, you will create a so-called "Slash-chord". Slash chords can sound awesome. You can even create odd chords by selecting a bass note which is not within the chord.
You can now select a different tone as a bass note. Initially, only tones that are part of the chord are displayed:
▶️ Select a tone that is not part of the chord
If you want to set a bass note that is not part of the chord, you can open the 🔒 "lock" to have every tone available.
▶️ Reset/delete a slash chord
Tap this icon to remove the bass note from a chord:
Q: I'm selecting a different bass note, but don't hear a difference!
A: Sometimes, the app plays the bass note in the same octave as the chord. Then you won't hear a difference. This depends on the key you are composing in, and on the octave, you have set in the musical preferences under "Auto voice-leading".