its basically, rotating until u find which players seem to work better with each other.

e.g I have 6 defenders, but a particular defence lineup from 4 particular players bring about better form from all 4 than if I picked any other players combination. It was a pain in the ass when I bought a new 18yr old with good training speed and tried to slot him in and "break" the quartet tandem. the defenders' forms all dropped and I squeezed a 1-0 win over a team in league that I had beaten 4-0 in previous match (my goalkeeper got his first MOM saving 5 goals lol, and GK form was 8). took 5 matches before the other 3 were back to form 7-8-7-8 and "got used" to the newbie.

I have 2 AML/ML, and 2 AMR/MR, the main team pair AML/AMR likes and assists each other more often.

last season, I had 2 strikers that performed when put up front together, and the form took a dip when I play lone striker up front like 4-5-1, they like playing in formations like 3-5-2, 4-2-2-2 etc.

for players that can play in more then 1 position, they have their favourite spots, like my AMR/MR playing better at AMR position.

all these little quirks aren't one-off things that happens in 1 or 2 matches, but there's a somewhat consistent pattern in some of these players. play them for about half a season and these little things start to emerge.