For one Tasmota supports the Home Assistant standard. Enable that and openHAB will automatically discover your devices and create all the Things for you. That would probably be the least amount of work.
Beyond that, Jürgen posted a good link for how to create the Things using text based .things files. If you want to manage Things through the UI, see Migrating MQTT1 items to MQTT2.4 items. For a generic MQTT 2 tutorial see the first half of MQTT 2.5 Event Bus.
Because you have a lot of devices, if you want to use the UI but don’t want to use automatic discovery, my recommendation is to:
- Create the MQTT Bridge Thing which connects to the MQTT Broker
- Create an example Generic MQTT Thing and test that it works
- From the REST API Docs query for that Generic MQTT Thing; copy the JSON that is returned
- Paste that JSON into the Thing Creation REST endpoint and edit the stuff that needs to be different (topics subscribed to, Thing ID, Thing name, etc.) and submit.
Repeat step 4 for each remaining Thing that needs to be created.