How to properly mount the bubble foil on the base, experiences?

You don't fix it, you just place it in between

You don't fix them, you just lay them in between

Sure? I've never heard that before

I would have seen it the same way, that it is not fixed. Ultimately, it is a mechanical protection that should also allow small movements. If you nail the conveyor down there, it will tear... "can" maybe, but definitely not "must".

Ok, so maybe fix something on the foundation and then perhaps press it against the facade with wooden slats until earth and gravel are applied?

Didn’t secure ours either. They were only about 30cm strips. Just laid something down briefly that was handy (paving stone or similar) and then filled with soil or gravel and done.