Kitchen project- queries on plumbing and repairs to subfloor

[Melter]

I'm in the process of ripping out our old kitchen and could do with some help on a couple of points.

First I'm trying to loosen the flexible hoses on the sink but the hose is just twisting and I'm not sure which nut I should be working on.

Second when I moved the unit next to the sink I revealed what you see in the pic below. The waste pipe is just sat in an open ditch and with some nice air holes in the wall where the pipe goes outside. The dark patch on the right looks like some hard set tar substance. What's best for tidying up this mess?

 

[Melter]

Okay sussed the flexible hoses so forget that one but welcome suggestions for the ditch in my floor. Cheers.