How the iOS can access to apple watch and airpod at the same time?
Bluetooth can only connect to 1 device of a type at a time.

For example, you can only connect to 1 pair of headphones (AirPods) or 1 mouse at a time.  You can connect to multiple devices (AirPods and a mouse) at the same time because these are different devices.

You didn't provide enough info on the devices you can't connect to, the errors you received (if any), or the troubleshooting steps you've already taken, for me to assist you with the "double connection" in your question.  Please see Help and then edit your question to include all relevant info.