DEC built everything in-house. PDPs, Vax, VaxMate, DecMate, Rainbow etc. were all internally designed.If I'm not mistaken, the DEC MicroVax computers also had OS, CPU and machine all made by the same company.
For personal computers, Rainbow ran CPM 86/80 (it had both processors) and Concurrent CPM as well as MS-DOS.
Vax and PDPs a variety of DECs OSs tailored to the task including VMS. Vax and PDPs also ran various forms of Unix and Venix. Vax ran a lot of BSD.
There were also other systems like the PDP10. But I personally did not have experience with those systems.
DEC had its own fabs, but might also have outsourced. X86 processors came from Intel, of course.