I know some of this, but not the whole answer.

One really important thing for helmets, carseats, etc is whether the material is multi-use or one-use.

One-use materials are generally far lighter for the same level of protection (protection itself a multi-faceted concept but I’m sticking with simplicity here).

Hockey helmets are generally lined with vinyl nitrile or polypropylene foam. Vinyl nitrile is the same stuff thats in HVAC gaskets, yoga mats, and all sorts of seals. It returns to its previous shape when you are done squishing it.

The inner foam of a motorcycle helmet is typically expanded polystyrene. It is designed to collapse and absorb force in an impact. Polystyrene foam is what many foam cups are made of (though obviously very different in structure in a helmet than a cheap cup). That foam has a bit of bounce and flex, but it is designed to permanently crush/collapse when it gets hit.

Both helmets have an outer shell that is designed to spread an impact over a large area.

But overall: multi-use vs one-use. Skateboard and hockey and snowboard helmets are multi-use (and weigh more for the same level of protection). You replace them when you take a massive hit that cracks the shell or sometimes after you use them a ton over time. Bike and motorcycle helmets and car-seats have to be replaced once they do their job - that foam does not work twice, but it’s much lighter for the same level of protection and used in applications where you don’t ever plan to actually need it.