How input and output multiplexers work in ADV7800?
Don't get hung up on block diagrams using exact terminology or internal symbols that somehow translate to a logic chip. Read the first multiplexer as having the ability to route any of the signals to any of the outputs. In practice not all options will be implemented because there's no point having a scart signal and (say) green (from RGB) being digitized simultaneously.

For the 2nd multiplexer read it as some form of cross-point switch - there are 4 inputs and 4 outputs - the real internal logic decides the order in which those 4 inputs are routed to the four outputs.