ECG

First, things you should know (This is what Apple says:)

• The ECG app cannot detect a heart attack. If you ever experience chest pain, pressure, tightness, or what you think is a heart attack, call emergency services immediately. 
• The ECG app cannot detect blood clots or a stroke.
• The ECG app cannot detect other heart-related conditions. These include high blood pressure, congestive heart failure, high cholesterol, or other forms of arrhythmia. 
• If you’re not feeling well or are feeling any symptoms, talk to your doctor or seek immediate medical attention.

It is not that the Apple Watch 6 does not support ECG in Slovenia ... but rather that Slovenia has not yet permitted Apple to release the ECG app and/or permit its use. Every country has their own rules and requires Apple (and other companies) to comply to these.

You will have to wait until your country allows Apple to release the ECG app and allow its use!

Specifically, to answer your questions:

Read these articles:

watchOS - Feature Availability - Apple

Taking an ECG with the ECG app on Apple Watch Series 4, Series 5, or Series 6 - Apple Support

What have you done up to this time? You've apparently monitored it in some other way. Just because this feature has become in other areas of the world doesn't mean it's now the only way to monitor your heart condition.