Visit https://emojito.meme/ you can create your own nostr custom emoji packs or add existing ones and then depending nostr client you use, you can react with them and/or add them to your notes like this :Boppop: . In case this doesn't render as custom emojis for your client, it is the way emoji is published to be displayed.

https://github.com/nostr-protocol/nips/blob/master/30.md