I board at a very basic place and know what my horse gets. The BOs have it set up so every horse is on roughly the same diet; everyone gets their own pile of X hay to munch on in pasture and that’s about it. Some of the breeding animals get concentrates when stalled, but not long term.
My mare is an eat keeper so this works out fine for her. If I had a horse who needed special feeding or something of this nature, I would expect to buy it myself but they would feed it.