Mroas, Optimal Mix and efficiency

[Emnlv]

Hi

With the MROAS we are knowing where to allocate the budget for a channel instead of another one, for the Optimal Mix. Higher mroas means we need to allocate more budget to it, right?

Here my questions:
Where, but how much?

Are we considering the efficiency of the specific media?
E.g. after a certain threshold of investment, channel efficiency can saturate, and become useless to invest in that specific channel

In addition, if we have negative mroas, in practical terms, we are saying that we need to avoid to invest in that channel, because is counterproductive?

As always, thank you for the support