The files go in the main file folder, respectively. So, for the jerseys it is:
C:\Users\NAME\Documents\Out of the Park Developments\OOTP Baseball 23\jerseys
The .oi files ship with the game and are associated with the current MLB licensed designs - so the .oi file format "locks" them from being edited.
From that uniform screen where I showed you the settings, you can also navigate to the folder the stuff is in and manually load a uniform, but the mod team usually has the name of the file in a format it will auto-load in particular years. So, if you want Cleveland to wear those garish, red alternate uniforms in 2022, you can just navigate to those uniform pieces and (at the bottom of that screen) lock them with a toggle to not update until you decide to change it.
It sounds more intimidating than it actually is
The .png files also work and but have the benefit of being editable. .pngs are the format that modders use when saving the new/tweaked/invented jerseys to allow them to be read by the game.