When a dog or other pet is euthanized the dog or other pet is first put to sleep with a sedative so that they do not feel any pain when they are dying.
Then the dog is injected with a drug that helps stop their heart and then they die within around 1 minute to 20 minutes but the dog is not aware of anything because they are sleeping and will not wake up once the other medicine takes effect and actually kills the dog.
That's a reason it's called putting a pet to sleep and it's a very peaceful way for the dog or other pet to die.
Dogs will usually suffer more when they are in bad health and are slowly dying so by having the dog euthanized gives them a more peaceful and less painful way to go.
Some people say that euthanizing a dog or other pet is cruel but what I find more cruel even though it's not anybodies fault is to let a dog or other pet suffer in pain and agony as they die a slow death.
I had a dog that loved to be outside even in the winter and she was old and gonna die anyway and she could hardly stand up or even see and had to have help eating.
I thought about having her euthanized but instead I decided to just let her stay outside on a freezing cold day and by morning I had hoped that the freezing weather would take her.
The good news is that the freezing weather took her overnight and by morning she was dead and her suffering had ended.
She got to be outside like she loved.
I would normally bring her inside during the cold but since she was dying she was okay to be outside.
She would've died regardless because she lived way past her lifespan.