I am just in the process of pulling a faulty one apart, faulty as in
not getting up to pressure so showing an error. They are quite
intricate for what little they cost - there is a tiny compressor,
pressure relief valve, over pressure safety valve, then a pressure
sensor IC unit, mounted on the PCB - which has the usual LCD display,
driver, buttons and rest of the electronics on it.

Then there is the pressure cuff itself, with two air ports, one for air
in, one going to the release valve.

My thinking is it is either the over pressure relief valve (which is
adjustable), pump, or release valve which has failed.

For the cost, not worth bothering to repair and a replacent is on its
way, but just interesting to see what was inside.