Demis D. John

For some built-in variants (eg. in 2021 version of the game), you first 1) Place the part on the build screen, then 2) right-click the part to show it's options, then 3) hit the arrow on the bottom-right to change the variant. Sorry I can;'t post an image to highlight the specific arrow - it's the bottom section of the popup window. For example, the "poodle" engine can be switched between double-bell and single-bell variant.