In my experience assists have been pretty random. I would say my wingers are more responsible for my assists historically as I usually set my wingers up to handle set pieces but there's never been like an open-play-assist-master I had on my team, and I run 2 DMC with playmaker ability (I literally train all my DMCs to have playmaker, its like a requirement to be in my starting XI lol) and they show up every once in awhile, but never to the degree of like a Modric who you can depend on to always thread it through. Again, AMC tend to be this person if anyone, but I feel its just the simulation and the position rather than them being actually good at assists. I just tend to keep my midfields attack stats above 100% especially passing so anyone could pop an assist (and half the time the game doesnt credit assists and count it as a shot the striker created on their own lol)