Hidden levels are left out of sorting, practice and progress. Your progress on them is kept.
Your progress lives only on this phone. Export a backup now and then, and always before switching phones.
Practice: swipe right = good, up = easy (or hold ✓). If you're sure, do it straight away; if not, tap the card to see the meaning first. Left = again, after seeing the meaning. Swiping a brand-new word up means "I already know it". ↶ takes back the last card.