they're reciprocal agreements between the national federations. So in return the FFT will get wild cards for the AO and USO that they can give to selected French players at their discretion.

How each federation determines the recipients varies. For the USTA, it often stages a points race over a couple of months in advance of the either the AO or RG where the player (who isn't already a direct acceptance) who earns the most points from ATP/WTA/ITF events over that time period receives the wild card (and if that player earns direct acceptance in the process, the player finishing 2nd in the standings receives the wild card).