Any number of them.
The generally accepted, modeling frugal, one, is to use a spray bottle of isopropyl alcohol (this breaks the surface tension), then to apply diluted (1:1 to 3:1) water and white PVA glue, typically through a dropper (although 3:1can be sprayed, it's just some clean up of the spray bottle). The spray bottles used are empty ones available at craft and hardware stores.
The technique can be seen in any number of diorama videos on YouTube (Kathy Mallett, Luke Towan, Laser Creation World, etc.)
Usually, any elevation change is done through a substructure built up using spackle, wall compound, culluclay/sculptamold, even plaster cloth. This is often built up over a base of carved foam, or similar materials, to "bulk out" the void space under a large feature.
The availability of closed cell expanded foam from various sources (product packaging, building site discards, home improvement stores, etc.) has made it a preferred go-to.
The old (now near-ancient) ways of building up papier-mache over wadded-up paper or the like will also work, if wanting longer drying times and patience.
I would recommend avoiding things like the ceramic clays as they can be extremely expensive in the bulk quantities wanted for decent-sized dioramas.