Infinite ammo in Resident Evil Requiem is not something you can simply enable from the menu. Rather, it’s a reward you gain towards the very end of your playthrough and turns your survival horror experience into a power fantasy victory lap once you’ve unlocked it.
To even have the option to enable this, you need to beat the main story once, after which you can access the Bonuses -> Special Content section, where Capcom puts all of the good stuff, including the Infinite Ammo (Guns) upgrade. This costs 50,000 CP and, once you’ve bought this upgrade, you can enable it across almost all guns in all your future saves.
The best way to achieve that 50,000 CP is not through mindless grinding, but rather through a ruthless second playthrough on the Insanity difficulty. Beating the game on Insanity difficulty alone will reward you with about 40,000 CP. Paired with the challenges you have already completed, you should have more than enough to buy Infinite Ammo immediately.
If you really want to break the game, consider pairing Infinite Ammo with Infinite RPG, which costs 35,000 CP, and Infinite Ribbons, earned by completing the game in under four hours with the Alpas ending. The former removes boss phases, while the latter removes the save management hassle that usually accompanies playing at greater difficulties.
On PC, you have the nuclear option of trainers/mod menus that lock your ammo count at the time you load in, but that’s best done offline, separate save, if you want a ‘clean’ completion.