iOS 12 has some very useful keyboard shortcuts yet. They’re especially useful for iPad users who have a keyboard attached. Here are the basic you should familiarize yourself with:
⌘ (Command) + H: Navigate to the home screen from anywhere.
⌘ + Tab: The quick app switching behavior you've come to know and love on macOS.
⌘ + Space: Open up Spotlight search. I frequently use this as an app launcher if I don't want to move my hands from the keyboard.
⌘ + Up/Down: Jump to the top or bottom of a website or other such supported scroll view, respectively.
⇧ + ⌘ + 3: Take a screenshot.
⇧ + ⌘ + 4: Take a screenshot and enter the editing UI.
⌘ + Option + D: Show and hide the dock.
⌘ + period: In certain supported applications, this kind of functions as an escape/cancel key.
When you are in any application you can hold down ⌘ (Command) to get a cheat sheet about what shortcut keys that app supports. You'll quickly find that most shortcuts are similar or identical to their macOS equivalent. Things like ⌘ + C for copy, ⌘ + V for paste and so on. Not all apps do a great job at supporting every expected shortcut key combination though.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)