>>Do they work like channel mixer sends?

Exactly.

An Assignable FX send is essentially a channel aux send and return built into one channel.

- Any number of Tracks can be "sent" to the aux send.
- The Assignable FX send "sends" to a chain of FX.
- The output of the FX is then "returned" and then routed/mixed .

A common application is to set up an Assignable FX with a Reverb (whet signal only) on it. You can then affect as many tracks as you want equally through the same "ambient" effect

Peter