Standards are documents, written by national or international standardization committees such as ISO and IEC, to document the "state of the art". They usually do not describe best practices or even "state of research". Rather consider the standards as minimum requirements that you as a professional company should not fall short of. 

Harmonized standards

a) Proving compliance

Some standards, the so-called harmonized standards, the EU has identified and published in its Official Journal. If manufacturers comply with these harmonized standards, you can go (eg auditors) on the assumption that essential requirements laid out in Annex I of the MDD respectively MDR are met. 

b) Examples for harmonized standards

The term harmonized standards include

• IEC 62304: Software lifecycle processes for medical devices 
• IEC 62366: Application of usability engineering for medical devices 
• ISO 14971: Application of risk management for medical devices 
• IEC 60601-1: Programmable, electrical, medical systems: basic safety and essential performance 
• ISO 13485: Medical devices - Quality management systems - Requirements for regulatory purposes

c) Problems with harmonized standards

As the European Community stopped the process of harmonizing standards, the standards, that are currently listed on the EU website, do no longer reflect the state-of-the-art. An example is the IEC 60601-2-12. This standard already was replaced by IEC 80601-2-12 (not harmonized). And if you read in Beuth.de, you will also see that the IEC 60601-2-12 is quoted as invalid.

More and more notified bodies insist, that manufacturers follow the latest versions of the respective standards.