The ECG does not detect my finger

A small percentage of people may have certain physiological conditions preventing the creation of enough signal to produce a good recording — for example, the positioning of the heart in the chest can change the electrical signal levels, which could impact the ECG app’s ability to obtain a measurement.

Liquid-free contact is required for the ECG app to work properly. Use of the ECG app may be impacted if the Apple Watch and/or skin aren't entirely dry. Make sure that your wrist and hands are thoroughly dry before attempting a reading. To ensure the best reading after swimming, showering, heavy perspiration, or washing your hands, clean and dry your Apple Watch. It may take one hour or more for your Apple Watch to completely dry.

For more info, see:

Take an ECG with the ECG app on Apple Watch - Apple Support

It sounds like you've tried a lot of things, it's probably time to contact Apple and/or send it in for evaluation and possible repair.

There is a "Contact" link on this page:

Apple Watch Service & Repair - Apple Support