Penguins are not really inherently friendly as in the way a domesticated animal like a dog or cat would be.
However penguins are not afraid of humans in most cases and may even be curious about humans.
Penguins also don't perceive humans as being a threat in the same way that they may perceive predators such as seals or orcas as a threat.
Although the penguins behavior towards humans will also vary by species and individual penguin, and some penguins are also more inquisitive and others are more wary of others.
Many species of penguins, most particularly the penguins that are in Antarctica and on islands with few land based predators, show little fear of humans and might even approach humans.
Penguins are also naturally curious and will also often approach humans, especially if the penguins are not acting in a threatening manner.
And penguins are also wild animals and should also be treated with respect and are not suitable as being pets and should not be approached aggressively or handled.
And just like other animals, penguins also have different personalities and some penguins may be more friendly and outgoing and other penguins are more shy and reserved.
Penguins can also bite and or strike with their flippers if they happen to feel cornered or threatened.
Although the penguins in zoos or penguins that are participating in encounters might be more accustomed to human interaction, although it's still important that you respect their space and observe any guidelines.