In one line
The attribution window is how many days after a click or a view a conversion still counts toward that ad. Click and view windows are normally set separately.
Where the numbers split
The same week's conversions can read 120 in Meta, 70 in GA4, and 85 in an MMP. None of them is broken — the windows, the timing of aggregation, and the conversion definitions differ.
Meta's default is 7-day click plus 1-day view. Layer in whether view-through conversions are included and the gap widens further.
Longer is not better
Widening the window raises reported conversions. But the added share is disproportionately conversions that would have occurred without the ad, which is exactly where reported numbers and incrementality move in opposite directions.
Go deeper
How to reconcile counts across systems is covered in attribution data mismatch.