First, the boards don’t sit directly on the conveyor belt. They are in carriers, and so their surfaces make no direct contact with the belt or anything else.
Now, what holds the backside components in place? In a single-pass reflow, two different things:
The key to making this work is the ratio of the component weight vs. its soldering surface area. If there isn’t enough surface area the part will fall off. That’s why in some datasheets you see a weight specification for the packaged part: you need this information to validate that ratio.
Further, some packages (LCCs in particular) add extra soldering pad area, not just for better grounding and thermal, but also to meet this weight/area ratio for backside reflow.
Knowledgeable PCB layout designers will keep this in mind, and if possible place larger and heavier components on the topside, reserving the back for smaller (lighter) stuff that's less likely to fall off.
If you instead choose a 2-pass reflow, then there isn't an issue. First pass (usually top) uses a higher melt solder; second pass (usually backside) uses a lower melt. This requires careful process control at the reflow stage.