To keep from being a parts jockey, please understand the following:
A microwave that is heating normally does not have a problem with the magnetron, HV diode, HV capacitor, door switches, or transformer. If any of those are bad, you will not get heating.
If the HV diode is bad, you won't get heat and it is unlikely that the internal fuse will blow because they usually blow open not shorted, but it might happen.
The transformer, magnetron and capacitor may blow the fuse but it is unlikely the transformer will fail and the magnetron is the lesser suspect.
Door switches on the other hand are the most common failure, either because they fail, or the mechanism that activates them is worn or misadjusted.
Door switches, mechanisms and adjusting (most newer MW eliminated adjustments) are simple for any mechanically inclined person with a multimeter. Diodes are the easiest to replace and cheap on Amazon (2x for $10.00 and cheaper on Aliexpress).
They are also easy to troubleshoot, since they usually burn up. If the magnetron is bad it is time for a new MW.
I am betting 98% of the MWs in the dump are simple, cheap repairs - Diodes, or door switches. And that is truly a waste.
HOWEVER, IF YOU OPEN UP A MW YOU MUST KNOW HOW TO NEUTRALIZE THE HV STORED ON THE CAPACITOR.