Each planning map comes with its own set of algorithms embedded within the map data to calculate which roads to use.

The algorithms are unique to each planning map.

After choosing the planning map you can also click Always use this map for new routes to set that map as your default planning map.

From now on all new routes will be created using the chosen planning map.

My screenshot below shows the planning map from HERE is selected and also the option to Always use this map for new routes.

The overlay maps are separate to the planning maps and have no effect on route calculations.
Think of them as clear sheets with extra information that you place over the top of the planning map.
The planning map underneath does all of the route calculations.

So with the planning map selected you can then use any of the overlay maps.
If you do not see the same blue menu bar with the planning maps that is shown in my screenshot, click Page menu first and then all of the same options will be available to you.