There were actually two separate American things.

First, Midwest had a frequent flyer hookup with American for several years in the "between Northwest" era. (YX/NW had FF deals in the early 90's after they dismantled the NW minihub and YX dropped MKE-DTW, and then YX/NW came together again in the mid 00's). The American deal was pretty much limited to you could spend YX FF miles on AA. Definitely no code sharing, no cross-airline earning, no elite-qualifying segments, etc.

The other American thing was actually a deal with American Eagle only. It was code sharing to something like 35 markets out of DFW and 12 markets out of LAX. By far the biggest two were MKE-DFW-IAH and MKE-LAX-SAN. (This came after YX ended SAN service.) But it also included YX* to places like Santa Barbara, Fresno, Fort Smith, Lawton, San Angelo, etc. Because it was only American Eagle, it did *not* include places like Austin or New Orleans because those were AA metal, not AA* metal. That deal lasted from the later 90's until about 2004 or so.

Personally I'd very much like to see oneworld / AA as the hookup, but that's just my personal wish.