It's very complex. Children are not property of anybody, but that doesn't mean parents are not responsible for them.
Also, respecting someone doesn't mean you agree with every decision they make or with every behaviour. However, people of older generations have lived longer than you and, in most cases, they have more life experience than you do.
Most parents also feed their children, give them a roof over their, clothes to wear, and other things that are considered necessities, and otherwise provide for them. It's their job to do it, of course, but children need to be respectful and grateful of that. Not take it for granted, not abuse it and definitely not act spoiled and demanding.
Many children nowadays get annoyed, angry and sad that their parents don't buy them a car, or expensive clothes, or a new iPhone every year, a new gaming console or laptop, and whatever other nonsense. It's not their job to do that and it's not a basic necessity. It's also not needed in any way whatsoever.
If they even buy you a cheap phone, you need to be grateful and not complain that it's not the latest model or whatever. If you want a more expensive phone, get a job and pay for it yourself.
How to discipline a misbehaving child is a complex topic. Many people don't have the awareness to understand how life works, and not the awareness to understand how to educate a child. You cannot always just talk to a child or teenager, though. Their brains aren't even fully developed, their whole bodies aren't.
It may be necessary at some point to use light physical violence, when children and teenagers are unresponsive to any other method.
Otherwise, like JG Karnonnos said, there obviously instances like yours where parents have unacceptable behaviour, and that's for the law to care of. This is not the norm. But also those people aren't fit to have children.
In the future, we'll have fitness to marriage certificates and fitness to have children certificate, with only mentally sane people doing those. Personally, I think that the worst cases of inept people need to be neutered altogether, so they can't have children to abuse.