If you want to use the classic co-term licensing or the per device licensing you buy the SKU's per model of MS switch or MX appliance. Only access points and camera's each have a SKU that is independent of the model of device.
For the MX you need to choose a license tier for your entire dashboard org. (exceptions are vMX and Z3/4 appliances).
For AP's you can mix and match.
So a network can have an MX with advanced security with switches that are enterprise and AP's that are enterprise.
In per device licensing you can mix and match between networks. So you can have 1 network with MR advanced and another with MR enterprise.
There is however the all new subscription license feature which binds everything per network. However this is quite new and I need to have a good read at that documentation. 😉