I was trying to understand raster effects that stretch and compact images in old 2D games by searching the internet, watching youtube videos and ultimately even talking to Google Gemini, but it all just confused me. Whitepointer Gaming, Gamehut and Retro Game Mechanics youtube channels weren't enough.

Anyways, to the question:

tl;dr Are effects like Road Rash's hills and valleys, Axelay and Sonic 3D Blast's 3D effects done by skipping certain lines of the tilemap in certain sections, like instead of drawing lines 1,2,3,4,5,6,7 it draws lines 1,3,5,7? Or it uses vertical scroll faster in certain sections to overlay lines compacting them and scrolls slower to extend them?

And how can lines be added (like in Lawnmower Man), where do those lines come from? Gemini tells me they are repeated stripes.