Both the iPad and Mac offer ways to quickly find features and their shortcuts. When doing keyboard-intensive tasks, keeping your hands on the keyboard is a great way to stay productive.
On any connected hardware keyboard, hold the Command (⌘) key and wait a beat. A cheat sheet of available features and their shortcuts should appear, and it might even have extra pages.
Note that this cheat sheet is contextual. For example: in Mail, different features will appear when you’re managing your inbox versus drafting a new message. Also be aware that not all iPad apps support shortcuts, though anecdotally, that seems to be steadily improving.
In any app, press Command + Shift + ? (⌘⇧?). This will open the Help menu for that app and place the cursor in its Search field. This is a Spotlight-ish tool provided by macOS that can search the names of all features with menu options in that app.
Type a couple letters, use up/down arrows to navigate results, and press Return to use the feature.