On a US English keyboard, the left and right square brackets are immediately to the right of the letter P.
Though I realize the original poster is not likely to be using a US English keyboard, remember that you can install additional languages in Settings, Time & Language, Language Pane. After you have more than one language on your Windows 10 installation it will support that language's keyboard "virtually" with whatever keyboard you may have on your system. I know someone who's a translator who routinely switches between keyboard languages on their computer. A new entry will show up in the system tray, which you can get to with WinKey+B and right arrowing over to hear the keyboard currently in use or change it. You can also use WinKey+Space to cycle through the various language keyboards for the languages you have installed.
I mention the above because while using Character map for the odd character now and again, and I do for the degree symbol, if you're trying to use a collection of symbols common on one language's keyboard or are typing something in that language switching your keyboard layout virtually can come in really handy.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back