Some information is available here:

https://dyatlovpass.com/sports-classificationAs I understand to be given a category as a hiker you need certain number of treks of the appropriate category.

The trek itself is certified, not your skills. The fact you cover the trek is the criteria.

See the highlighted text in the article:

For example, for the third ski category (the one that was planned for the Dyatlov group), it was equal in total length to at least 300 km, of which at least 100 km they had to go with overcoming natural obstacles i.e. steep uphill climbs, steep descents, tracing (making a path in deep snow) ski tracks, moving through a dense forest with undergrowth, moving in a treeless area far from villages, etc.

The total duration of the trip was to be at least 16 days, including in a completely unpopulated area - at least 8 days. This means that the distance to the nearest village should be at least two large daytime crossings - at least 50, 60 km. Moreover, they had to make at least 6 nights completely in winter conditions. This means that they had to fully organize the night with only the set of equipment that they brought with them. The overnight stay in huts, shelters, or abandoned houses should not exceed 10 days out of 16.