Don't confuse solute with dispersed phase and solvent with dispersion medium.(These are not same things).
But only for the sake of easy remembrance. You can call dispersed phase as (solute) of heterogeneous mixtures. AND Dispersion medium as (solvent) of heterogeneous mixtures
The solute are invisible particles due to their extremely small size after dissolving in a solvent.
That means before becoming salt and water, the salt and water are two separate things but after dissolving salt crystals disappear and uniformly distributed throughout the water being invisible giving a clear salt solution (even though it still contains 2 things)
Whereas in case of heterogeneous mixtures two parts Dispersion medium and Dispersed phase, are visible distinctly.eg- oil globules(d.p.) in soapy water(d.m). That's why we use term Dispersion (scattered nonuiformally and are visible) phase(distinct part) The particles are smaller for Colloids and bigger for Suspension.