Yeah, the diffusion does depend on where it's placed and also what implant type (N/P) you use. But as the diagram @User shows in each well type you can create P+ or N+ diffusion regulations. In terms of making transistors there's the PMOS/NMOS included in the PDK with various threshold voltages, maximum Vgs and Vds voltages. And NPN + PNP transistors so it's definitely possible. The vertical NPNs uses that sandwich structure as does the vertical PNPs but their collector is PSUB so of limited use.