Spacefolder ships use the Holtzman effect to bend spacetime. Computers were used to find safe routes before the Butlerian Jihad. Without computers, guild navigators use their prescience to find safe routes. Spice doesn’t allow spacetime bending, just navigating through bent spacetime without a high fatality rate from flying into stars.

If you watch really closely in Dune P1, you can see another planet (presumably Selusa Secundus) through the center of the navigator ship. I don't know how it functions mechanically in his version, if they're navigating wormholes somehow or controlling them, but they have some form of wormhole mechanism shown in the series.

Edit: The image embed didn't seem to work, so here's the link to the image. https://www.reddit.com/r/dune/s/pSBHvR3Ser