DA and MDA definitions are in the books (PANS OPS, I guess)
Adding 50 ft stems from the "ILS like" Non-precision approaches. These are flown with a constant descend angle (of about 3º) so that when reaching the MDA, you can land if you see the runway because you "are in the glideslope". If you don't see, you don't stay at the MDA waiting for the MAPt to see if you can land or have to go around, but you go around inmediately.
But doing this means that you will sink a few feet below MDA, which is forbidden because MDA is a minimum altitude. That is why they add the 50 ft, to cater for that.
Also remember that the MAPt is a point of the non precision approach but it is not a decision point. It is just the point at which the procedure starts, so you don't make any turn until overflying it. Ideally the MAPt should be the runway threshold, so it is a bit difficult to land if you become visual over the threshold at, say, 350 ft!
NPAs flown in the old "dive and drive" style are non stabilized manoeuvres.
They can be flown "ILS like" with a VNAV system or just following a distance-altitud table. You can make that table yourself, but that can be very tricky, a lot of different mistakes can be made when calculating it. It is better if your chart has that table.