For the most part you can specify that the miles accrued from a flight on any OW airline are to be credited to your FF account with any other OW airline. And in general you can use the miles that you accumulate in that account to buy trips on any OW airline.

But it's so far from a "common currency" that I'd say my first two sentences have no practical meaning.

The mileage credit you get with your "home" airline "A" when you fly a segment on airline "B" varies from zero to full credit or more, depending on route and class of service among other things. The cost of redeeming miles is even more variable, I think.

So I think most people simply sign up with an airline that they're likely to use a lot. The best accruals and cheapest redemptions will always (I guess) be on your "home" airline. If you do a lot of flying and are likely to put a lot of miles on more than one airline, then it's worthwhile to look at the details of those two or few airlines to see how to manage your miles.