DPS in the millions, how do people calculate it?

Many of them are just made ups.

Most of them can't ever sustain that "millions of DPS".

They activated all enemy debufs in Path of Building, and most of them can't pull it off in boss battles with very few adds.

Most don't even have a propper setup to maintain frenzy charges.

Most people just click every button in PoB and represent that the figure stated is accurate. Clearing DPS w. flasks up 100% and on kill effects, etc. you need to build in a dedicated manner to have this. Nearly nobody does. Then you have to look at boss/shaper DPS. That they have these flasks up 100% to have this # against shaper, this is inaccurate, this is misrepresentation.