It usually exists in 2 ways. First, you have the sidebar button which should be visible on your top right, unless you removed it. Like for me:
But if you went to Settings → Appearance and chose to remove it…
Second way is through keyboard commands. Such as Ctrl + B is the Toggle Sidebar command by default. Of course, you could go to Settings → System → Shortcuts and change the command if you want.
All of that said, I will tag @Mattches on this because it used to place the Sidebar button back on your UI when you toggled it, then vanishing once closed again. I’m not sure they intended it to work as it is, where people might have a more difficult time closing the sidebar if they had chosen to hide the icon. But if you have the button disabled, it doesn’t show when you open. So you’d have to know the keyboard command or change the settings for it to appear again.