Flooring – How was this subfloor cleaned

I'm replacing the floor in my bathroom, which was previously a vinyl sheet. Prior to that (and I think original to the house, built in early 70's) it was some peel and stick-type tiles, except they're very hard (they remind of me what would be sold as commercial tiles now). There were some tiles left under the old vanity, and since I'm replacing it with a differently-sized vanity, I figure I'll put the new floor underneath it as well, which means the subfloor has to be flat.

After I scraped up the old tiles, the floor is all black, presumably from the adhesive in the tiles. The rest of the floor though, that was covered by the vinyl sheet, looks like it's had the adhesive cleaned off.

How was this cleaned, and why is it all white now?

The floor was cut exactly to the outline of the vanity, and from the old paint on the wall, it looks like prior to the vanity there was just a wall-mounted sink. I'm going to guess the vanity was installed, and then sometime later the vinyl flooring was installed, as otherwise I can't see how the tiles got cut to this pattern.

I'm putting a floating vinyl plank tile floor in place, and this part will be underneath the vanity anyways (the new vanity is slightly bigger). Should I bother trying to clean this part as well, or just put the new floor over top?

Update: I added a whole bunch of screws to reinforce the floor to the joists below, and help reduce some of the squeaking. In one spot, right along an edge of the plywood, the white stuff flaked off – so it seems rather than being 'cleaned', there's actually a really thin layer of something on top of the black adhesive. There's no noticeable difference between the height of the two sections.

I've since laid the new floor on top, I didn't do anything extra to this subfloor.