Since the restructuring, it is Alphabet that is publicly traded. As a result Waymo is part of the publicly traded entity equally to Google. You can already invest in Waymo by investing in Alphabet.

That is true but what usually then would happen is successful businesses are then spun out and they do their own IPO.

Alphabet would retain controlling interesting. This is pretty common but not the only way. Sometimes parent company shareholders are given shares in the new company directly. So say every share of GOOG might be given a share of Waymo Inc for example. It is not always the case that the parent company keeps control.

It can also happen the other way where a sub is public and the parent does an IPO.

The structure of Alphabet is what gives the flexibility to do this.