Does cape use its own cell towers, or do they rely on third parties to provide the actual infrastructure? And if they do use third parties, are they sure that they aren't also storing data about the connected devices etc?

We don’t operate our own towers and as you point out we can’t control what someone there does. Our privacy and security model is to treat the towers as untrustworthy. This is why we do things like rotate your IMSI daily or split your traffic across multiple underlying network partners. We want to make any data that is collected noisy and less valuable to data brokers.