Before the jam starts, I just want to ask a quick question — how will we upload our projects? Do we need to upload them to our Itch.io page, or submit them directly through this link after the jam starts: https://itch.io/jam/turbowarp-game-jam-25?

you upload them the normal way you submit games to a game jam. so (i think) you need to upload your game to itch.io, following the guidelines mentioned in the description, then you select your game at the jam page.