Strange as it seems but the manufacturer doesn't get much say in how his aircraft product is maintained.
The Regulator has that luxury.
The Regulator, CASA, FAA, CAA etc gets to state how their rules are to applied.
Generally it would then be iaw the manufacturers recommendations to a greater or lesser extent.
For FAA aircraft they would generally require 100 Hr Inspections (can be by an A&P) with the Annual to be done by an IA. The IA has to approve mods and major repairs also.
The A&P certificate has not always been held in high regard by licenced engineers in some counties. A little bit like a watch with a round black nose and big ears.
Until recently it really did look like it came on the side of a Weetbix packet.
You had to cut the cardboard along the dotted lines, - literally.
The FAA IA would be a much more useful and valued document.