Google Ads Conversion Adjustments

I’m starting to get my hands on Google Ads Conversion Adjustments

I find that everything is clear except for a few information.

How do I retrieve the user data (ENHANCEMENT) so that Google can merge the data?

I should create a variable that gets the value, otherwise I should create tags for each individual request.

I’m not sure I understand your question correctly.
In the incoming request on which you want this tag to work there should be user data, you take this data through variables, add it to the tag and thus when triggered it will be sent to the specified conversion action id.

I’m just starting to get my hands on this configuration and I wanted to understand if just
Google Ads Id, Order Number was enough or if other information was needed too.

I would like to be able to send the payment change from pending to paid from the CRM.
Thanks

Yes, the order number is sufficient for identification, the same one you send when the initial conversion takes place.