Can you tell me what is meant by ‘build’, ‘descend’ and ‘negative split’. In the main set, I have 4 x 50 or these with a 20 sec. rest interval.

Of course …….

“build”….”descend”….and “negative split” all have similar meanings.

“Build” typically refers to the energy or effort you put into the set.
The amount of energy or effort “builds” throughout the set.  Example
on a set of 4×50 on :20 rest.  #1=70% effort.   #2=80%.  #3=90%.

“Descend” refers to time.  The time you are able to achieve on each
swim within a set progressing improves.  Example on a set of 4×50 on
:20 rest.  #1=:40.   #2=:37.   #3=:34.  #4=:31

“negative split” refers to a swim where the second part is faster than
the first.  In the case of 50’s for each 50 the second 25 (or 2nd lap)
faster than the first 25 (or 1st lap)

I hope this help.  Enjoy your workout
Never train alone again