The main terminology that must be exposed is the differentiation between purification and isolation of alleged viruses.

These words are frequently considered equivalent, but I considered them differently:

Purification: to separate alleged virus particles from everything else;

Isolation: to demonstrate alleged virus existence, that is, includes to purify, but involves other steps too, including that the particles replicate.

A virus existence could be also determined with indirect observations, mainly the so-called genome sequencing. So this would be an isolation too.

But there are certain doubts about alleged genome sequencing as a valid method to demonstrate viruses existence (see here)