On MacOS as well as iOS : is it possible to search directions including an intermediate point ("via") ? Google Maps can do it.
Yes, you can do so in iOS Apple Maps, but you can only add intermediate waypoints in certain ways. Specifically, you must have already started navigation to a destination. At that point, you can add pit stops along the way. Once you've reached a pit stop — an intermediate waypoint — you can then add another. You can keep doing so until you reach the final destination.
Once you've entered the Navigation view, you can either touch/tap the screen (if you're not driving) or use Siri to add intermediate waypoints.
Via the screen: swipe up from the bottom, choose an intermediate waypoint category from the offered list (things like coffee shops, gas stations, etc.), and then tap "GO" on one of the specific locations. The location will be added as an intermediate destination on the way to your original final destination.
Alternatively, you can use Siri to add an intermediate destination. While in the navigation mode, engage Siri with "Hey Siri." Then tell Siri to "add a gas station" (or other type of location). Siri will present you with some options and allow you to choose among them with your voice. Once you've chosen location, tell Siri you want directions and Siri will add it as an intermediate destination on the way to your original final destination.
This link has some good details on the topic: https://ios.gadgethacks.com/how-to/apple-maps-101-add-multiple-destinations-your-directions-0181465/