The working assumption is that the market makers / hedgies take the other side of virtually every trade that retail makes, and that mostly means selling of options.
Using that 750 strike in particular, we're assuming that MM are the primary sellers of those puts and retail / hedgies / whoever are the primary buyers and owners of those puts. To the degree that the market makers can control the share price they would like it to be somewhere above 750 so that those 750 strike puts expire worthless.
The hedgies don't care if the shares go to 740 and we lose money - they care that they've sold a LOT more of the options in that lump and they don't want to lose money.
Of course if we're wrong and you and I are the primary sellers of those 750 strike puts, and they've been purchased by the market makers, then they would like the share price to finish below 750.
This gets more complicated by the reality that not every market maker or every hedgie has the same position or motivation, any more than you or I have the same position and motivation. The OI and put wall concept is an aggregate concept - there are MM above that in their own position, MM below that in their own position, etc.. with the sum being that put wall.
I recommend looking at the tables below the headline charts at the max pain website. In the second table you can see the put loss, call loss, and the combined loss at every strike. Max pain has been described elsewhere as a bathtub - there is a pretty big and wide bottom where the outcome is about the same for the universe of option sellers (which we believe to be primarily the MM - this is why we sell options; Be The House). Not every option seller has the same motivation (position), and not every option seller has the same ability to move the market, but its an overall result.