Why are purebred dogs all inbred? Are all dog breeds inherently inbred?

I watched a video that said dog breeds today all insane health problems. I was concerned, but why is it like this? Breeds have existed for a while, how long has it been like this?

How are dogs being bred now that they weren’t before? Are all purebred dogs inherently inbred, like it’s required to make them?

Which dog breeds aren’t inbred? I was looking for a big gentle dog like a burmese or st bernard, but apperantly they are subject to this too, why is it everywhere?