Ingest⌘ is Command, ⌥ is Option, ⌃ is Control, and ⇧ is Shift. A key written on its own, like T, is pressed on its own.
Some keys mean different things depending on where you are. In the file browser, there are three places: the grid of thumbnails, the large preview, and a video playing in the preview. Where a key only works in some of them, the table says so. Apple Photos has contextual commands too. Decks is a separate app.
Single letters pause whenever you are typing. If the cursor is in the keyword field, T types a letter t. Press Esc to leave the field and the keys come back.
A command that cannot run right now does not fail quietly. It beeps and the status bar says why, for example that nothing is tagged, or that the source is read only.