Sidecar was for Amiga 1000 [1]. For Amiga 2000 there was the somewhat simpler Bridgeboard / Janus, which plugged into both the Amiga Zorro bus and a separate ISA bus in the A2000, and kept the whole thing internal, and let it access hardware on both buses (mostly used for IO, and to show the video output from the PC side in a window on the Amiga.

The Sidecar plugged into the expansion port which was basically mostly the internal bus just exposed as-is.

[1] https://en.wikipedia.org/wiki/Amiga_Sidecar