Yes - a bike cargo rack.

You have a waterproof bag that attaches to it. Problem completely solved.

It rains quite a bit in Netherlands, and it's the biking capital of the world.

It's almost as if rain isn't really that big of a deal.

Baskets, racks, panniers, backpacks, delivery services, cargo bikes, trailers.

A significant number of Parisians don’t own cars so this is no different than before for them.

Even without a cargo bike, I carry 20 to 25kg of groceries using a standard step thru city bike equipped with a front and rear rack and dual bags front and rear + backpack.

In Paris, you will see a lot of uber eats/deliveroo/etc. drivers on bikes and electric mopeds. It reduces their costs by orders of magnitude while not really changing their delivery times (probably shortening them actually. Unrelated to the question, they drive extremely dangerously (because of the way they're paid, they're doing all they can to cut times. do not expect them to stop at red lights))

They have those huge backpacks [1]. Some fasten it like a kind of top-case.

[1]: https://images.app.goo.gl/XVyoMqpQiQFvC5EU8

Yes, there are entire companies devoted to making them. Here are a few:

https://www.revzilla.com/motorcycle-top-cases?gclid=Cj0KCQiA...