
I think it varies..
Some boarding kennels occasionally get landed with a dog they need to rehome - some win the contract for the local authority to be a pound as well as a commercial boarding outfit , and do some rehoming themselves - some people run a rescues alongside a boarding kennels perfectly legitimately - some run a second hand dog dealers alongside a commercial boarding kennel - some run a pretend 'rescue' which is nothing more than them trying to avoid a commercial breeding licence....
And then for all of these possible permutations, some people do it very well, with total transparency or excellent animal welfare standards, and some people do it absolutely abysmally and of those, some people have no idea they are doing badly, and some people do and couldn't give a rats ass.