Sure, you can tap the More menu at the end of each App Extension row and drag them up and down in a list. But who has time for that? No one, that’s who. You can long press, then drag-and-drop them left and right in their respective rows.
Generally, most apps should respect the custom order you set for extensions, but I’ve run into some cases where they don’t. I can’t find the rhyme or reason, and I’m not sure if it’s a bug or something developers can deliberately do to make sure their actions appear first.