I read somewhere that the bass line is usually the tonics of the chords in the progression. But I doubt this is right
If you did a statistical analysis across pieces of music that feature an identifiable bassline and an identifiable chord progression, you'd certainly find that the bassline is playing the root of the chord a lot of the time. Whether it would be 'usually' - as in, more than 50% of the time - I'm not sure. Some basslines mostly stick to root notes, some mostly stick to chord tones, and some basslines use lots of passing notes which aren't necessarily even restricted to the diatonic notes - walking basslines in Jazz are an example, but there are lots of examples where rock and pop songs do this.
As suggested in Bass line melodies, a bass line doesn't have to serve a chord progression at all - it can be the centrepiece of a song, with other elements working 'around' it. Often songs of these type don't have a clear chord progression, even though the motion of the bassline and other elements still gives a sense of motion in the harmony.