Gantry with grav: where do page templates go?

I’m building a first site with grav and gantry5, using helium as starting point. I’m still unsure of what to do in gantry vs grav. My impression was that user/pages hold the content, while the presentation is in twig templates under user/[theme]/templates. The latter is absent under helium.
Is there another location for page twig templates under gantry5?
Or am I supposed to use outlines and/or particles to define the presentation of page objects, instead of page twig templates?
I guess I’m missing info on how to use gantry with grav specifically.