Help me with uploadding multiple files and run a loop

I'm developing a shiny app, trying to upload multiple files and then run a specific loop for a simulation. All code already exists, I have been able to upload the files into the shiny, but then I'm stuck totally. The whole procedure of the loop and simulation is already done and reproducible outside the shiny, in the console, so it shouldn't be that complicated. I would be grateful to get help with this.