Having worked in several Veterinary clinics across the country I have seen what they do with pet remains. It is not pretty, in fact it is quite disgusting and heart breaking
Simply put, IT IS NOT ILLEGAL for euthanized or deceased animals to be sent to a landfill. It is actually very common and more of the norm than actual cremation.
Deceased pets are placed in bags. Sometimes these are bio-hazard bags and sometimes these are regular garbage bags - there is no law dictating the choice. The pets are frozen and held until someone from the landfill is called to pick up the pets, usually when the freezer starts to get full. The pets are loaded and taken to a specific part of the landfill. They are dumped and covered. Beyond that, I wouldn't know.
It's a horrible thought and terrible to think about.
There are of course optional services, such as cremation. I however have never heard of a vet offering free cremation. If you have a vet that does it then you are very lucky.
Words of advise, beware of the words "communal burial", unless you are paying for cremation and for the ashed to be spread over rainbow bridge. Communal Burial is only a politically correct and gentle way of saying that they will dump your pet with other pets, in a landfill.