Unlike wired earbuds, AirPods don’t connect in any physical way. You can use the find my AirPods feature to find them if you misplace one or both, but only if they have enough battery life to sound the alarm. If you lose an AirPod and have given up ever finding it, you can buy a replacement from Apple. However, it won’t work with your old AirPod right out of the box.
You can use two different AirPods together even if they weren’t originally part of a matched pair, but only if they’re the same type of AirPod. You can’t use an AirPod 1 and AirPod 2, or one AirPod 2 and one AirPod Pro. They have to be the same type and generation, or they won’t connect and work together.
To connect a replacement AirPod to your existing AirPod, you need to reset the original one to work with the new one. Resetting turns the old and new AirPods into a matched pair, and you can then connect the AirPods to your iPhone.