Appreciate any info on where Gates A8 or A12 in Schiphol airport are located. Are they real gates or airlines like KLM due to crowded terminals instead of keeping passengers waiting inside arriving planes, de-embark them on tarmac and transport them by bus to some terminal?
Because Schiphol is intensely used, arriving planes sometimes cannot approach a terminal gate because those gates are already occupied by other aircraft. When this happens, arriving aircraft are parked at remote parking stands on the airport grounds. The remote parking system is described on this Schiphol Factsheet
This Schiphol airport webpage, titled "Airport Process Maps," contains links to further maps of the system; three are relevant here:
"Overview Remote Aircraft Stands" shows the location of existing remote aircraft stands, grouped into "Platforms." Note that there are no Platforms labeled "A."
Page two of "Overview Bus Gates" lists the portals used for remotely-parked aircraft. A8 and A12 appear on this list as "Arrival Bus Injection Points." (The word "Portal" is used in some documents.)
The map "Overview Gates" shows the locations of both A8 (between regular Gates D5 and D7) and A12 (near the base of the F pier).
Thus, A8 and A12 are bus portals in the existing terminals. Through them, passengers access buses that take them to and from remote Platforms. The specific airplane locations could be on any Platform.
Schiphol does not have a Terminal A, so there are no conventionally-numbered Gates A8 or A12. A new terminal is currently under construction next to Terminal B, and is expected to open in 2023.