Whales can be someone one who had invested quite a lot of money in crypto, when it was still very early stages. As a result their wealth has ballooned over  the last few years, even if crypto is a fraction of it was at its height these people are still well in the green.

There isnt really a "who" its just a subsection of people invested in the sector. Each coin has its own set of whales that may be only relatable to that coin/token and are not considered a whale in anything else. For example, someone who got in very early in an ICO or someone who has been accumalating BTC over the last 10 years, just cause they are a Whale in the token or BTC doesnt mean they are a whale in the other.