I’m started shooting last month. Can anyone help me understand why this roll of Portra 400 came back so grainy?

I didn’t record the settings I was using at the time unfortunately. I’m pretty sure I had ISO set to box speed, but I could be mistaken I guess? I’m still trying to wrap my head around ISO/aperture/shutter speed and how they all work together, and I’m also still getting the hang of rangefinding (shooting on Yashica Electro 35 GSN), so I’m just generally confused as to what would be the most likely rookie mistake I made here. Any insight you have is appreciated!