In which version does this grid/list togge icon exist? It's not in Ubuntu 20.04 LTS (which we use at work at the moment).
In my Ubuntu 20.04 LTS, i see the grid/list toggle icon in the top right just to the left of the minimize/maximize/close icons on the top bar of the File browser window. Once you click that, you can adjust the size of the thumbnail(s) by CTRL-scrolling or CTRL-+/-
That must be a different window - the file browser. The open file dialog doesn't have close buttons, so you're not looking at that. I believe the main discussion here is about the open file dialog, the "file picker".