These are the mythical mountain stages where the spectacle is most appreciated and where there are the most people. Several hundred thousand people during a stage that arrives at Alpe d'Huez for example, where the supporters gather several days before and transform the side of the road into an open-air discotheque.
It's an experience you won't find anywhere else. If you can find a place.
Otherwise going to a stage arrival city is more interesting than to a departure city
Or you have to find a small town or a village on the course of the race (located in a climb).
In most towns or villages where "Le Tour" passes, it's party time, with music, beer stands, sausages / french fries / mustard (or ketchup if you prefer! )
Next month the places chosen for the start and finish of the stages will be published (we already know the details of the first 3 stages which will be in Spain)
The details of the other routes followed by the race will probably come later.
The first thing to do when you have decided where you want to see "Le Tour" is to find accommodation, and to worry about possible accesses from this accommodation because all the roads which access or cross the route will be closed several hours before the passage of the race.
If you have a car it will also be impossible to park it on the race route one or two days before the race (if you forget it, it will go to the pound)
And be aware that places to stay will not be easy to find, not only because of the tourists, but the Tour De France is a real city of several thousand people who move from city to city every day ( I was one of those people for a few years) and for whom accommodation is booked well in advance.