Estimation algorithms for Multi-User MIMO

I am interested in a question how to estimate a channel/symbol in a multi-user system. In the case of a single-user, it can used ZF/ pilot estimation/ MS/ LS algorithms. Do we have a difference in using these algorithms if we consider a multi-user system?

In the multi-user case, one signal is transmitted via some number of channels ( = number of users). As a result of the estimations algorithms, we get some number of detected signals and channel estimates.

If there is no difference in estimation algorithms, why are algorithms for multi-user studied as a separate issue?

in case of estimation of the transmit data ( e.g ZF), there are different channel, it means ZF gives different estimates of the transmit data, doesn’t?