Yes they clean the system internally. This stops further damage and future breakdowns related to sludge.
It will depend greatly on where in the boiler the block is and the boiler layout. Some boiler makes are easy to flush clean, other cannot be flushed clean at all.
Yes, the amount depends on on how much sludge it had in it before the flush.
Sludge is a cumulative name for rust and debris found in your central heating system water.
If you have a real sludge problem “NO”. Sludge is metallic rust and the bits causing problems are large clumps that are heavy. Draining a system down will only get rid of the finest lightest sludge; the real problem will be left behind. Just because the water runs clean does not mean all the sludge is out. Some plumbers will do a chemical flush; they add a chemical, days before draining the system down. It doesn’t work because there is no increase in the water’s flow rate and draining happens through just one flow path, when there are many flow paths in the system. It will help a little if the problem is small but 90% of the time, you are wasting time and money.