The mounted combat rules say that you can either control a mount, making it move on your turn — but its action can only be Dash, Disengage, or Dodge — or have it be independent, meaning it can do its actions but you have no control over it.
Now, if a ranger uses their Beast Companion as a mount, could they direct it as they usually do when unmounted and still have its full array of actions, since the beast still acts on their own initiative?
The revised ranger says:
The companion obeys your commands as best it can. It rolls for initiative like any other creature, but you determine its actions, decisions, attitudes, and so on.
And independent mounts says (PHB p.198):
An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes.
These don't come into conflict so a ranger (or, indeed anyone the ranger wants) can ride their companion as an independent mount and they can direct the mounts movement and actions - in effect "it wishes" to do what the ranger directs.
