My understanding is that they can/do on WhatsApp.

On Signal, unless there is a some bug or outright fraud, afaik they cannot - that is one of their fundamental goals, and they did a lot of work to develop communication technology that worked without revealing that metadata.

(Of course, if someone gets access to your phone, then they know who you are talking to.)

That kind of meta data is not stored by signal as far as I know. But yes, data stream between two end points can be linked to communicating with each other.