How to find Firebase users not in 'Username-Password-Authentication' with 'Database Action Scripts'?


I would like to ask you for simple examples to understand the use of “Database Action Scripts” in order to find an existing user in Firebase, in case this person is not already listed in the “Username-Password-Authentication” database.

If it is possible, could you help me to clarify this technical point? I know my request is specific, but I hope you can help me with this. Your input would be greatly appreciated and I thank you in advance for your time and attention.