>I'll go first
If two fluids have different densities and the particles have the same mass then the fluid with lesser density has particles wich move larger distances in a given span of time while the others move less. The denser particles act like a cushion from which the other particles bounce, pushing them upwards on average.
If the fluids' particles have different masses and are both at the same temperature then the more massive particles tranfer more speed to the others due to conservation of momentum, which in turn promps the same effect I mentioned earlier. The denser fluid acts as a cushion for the other to bounce.

I could be entirely wrong since I'm an undergrad but this is the explanation I deviced when normies ask me about buoyancy