Who cares? The point is that it's their problem, you don't need to think about it, it's done for you. =)

In practice your application restarts at least once a week. It's transparent because a new instance is started first and takes over. The provider can move applications around to add/drain servers and perform maintenance.

AFAIK Dyno is alive for 24 hours or less +-1h, not a week.

You are indeed "on their schedule" -- see eg https://devcenter.heroku.com/changelog-items/1603. But that's the trade-off you deal with, so that you don't have to manage all this stuff yourself.