Hamburger menu is the three dots, where you access your settings within Brave. In larger browsers it would be three lines on top of each other. They call it a hamburger menu because it’s kind of like a hamburger between two buns…just a silly term.
In this screenshot of Brave on my iPhone, you can see the menu on the bottom right. I forget it that’s the default location or if I switched it up, but pretty sure it’s default.
When you click on it, you’ll have all your various options there, such as Bookmarks, History, Downloads, Wallet, Playlist, Night Mode, etc. As 289wk said, Request Desktop Site will be towards the bottom, with two options beneath it (so third from bottom)