Hover any message to see a quick-reaction row of common emojis (👍 🎉 ✅ ❤️ 😂 👀). Click one and you've reacted. Your most-used emojis bubble up into that row over time.
The full picker
Click the smiley face in the hover row (or the + on an existing reaction) to open the full picker. It's organised by category, has search, and remembers your recently-used emojis at the top.
The same picker is used when you set your status emoji — recently-used is shared across both.
Removing your reaction
Click your own reaction chip to remove it. The count drops; if you were the only person, the reaction disappears.