Hagrid told Harry when he took Harry to Diagon Alley for the first time that no one knows why he survived the killing curse and that is why people called him:

“The Boy Who Lived.” Though perhaps Albus Dumbledore knew why Harry had survived.

As Albus Dumbledore knew that Severus Snape had asked Voldemort to not kill Lily and Voldemort for some reason agreed but Severus Snape didn’t trust Voldemort.

So, this is why Severus Snape went to Albus Dumbledore for help and he agreed but Albus Dumbledore told Severus that he was disgusted.

As Albus Dumbledore told Severus Snape that he didn’t care about Lily’s son or husband as long as he got what he wanted.

It is also known that Albus Dumbledore spoke to James Potter and Albus offered to be the Potter’s secret keeper, but James preferred Sirius Black.

However, Sirius declined, and Peter Pettigrew ended up being it and Voldemort somehow found out as Pettigrew revealed their location to Lord Voldemort.

Although Lily did have a choice to live however though she had to let her son die, but she didn’t want that.

Also, by standing in the way and being defenseless she ended up dying.

Then this led to a sacrificial charm being cast (an extremely rare occurrence) and when Voldemort tried to kill Harry the curse rebounded.

Voldemort also unknowingly made a horcrux. As Harry was still alive and Voldemort was too as he had made other horcruxes.

However, Voldemort was now changed into something else, and he then fled the scene.

Albus Dumbledore somehow knew that Voldemort hadn’t died, and he told Severus Snape that there are ways that Voldemort could return.

Then Severus Snape agreed to help Albus Dumbledore.

As the headmaster told Severus Snape that Harry needed to be protected.

He also told Severus that Harry will be in danger when Voldemort returns. Though Harry had to live with Vernon and Petunia Dursley.

As Petunia was Lily’s older sister and Harry’s only living relative.

Also, Albus Dumbledore wrote Petunia Dursley a letter when he left Harry on the doorstep.

Then even though the Potters and the Dursleys didn’t get on the Dursleys allowed Harry to live with them.

As perhaps the Dursleys did this because of whatever Albus Dumbledore had written in that letter.