How did the terrorist’s breach the cockpits?

I know this has probably been discussed before. So I hope you don’t mind me bringing it up.

I often contemplate how they got in. The hijackings were very planned & co-ordinated, so I’m guessing that each group of hijackers were drilled to do the same thing/s.

Using the flight attendants seems to have been part of that plan. On the UTD93 ATC recording when the first mayday call is heard, a female is heard saying “oh my god, stop”. Her yell sounds like she was shocked & surprised it turned deadly & was (understandably) horrified by what she saw. That gives me the impression she may have been used to open the door. However on AAL11 Betty Ong said the “guys must have jammed their way up there” when referring to the hijack. Betty may not have seen first hand but I’m guessing first class passengers who’d run back to coach would have done & what they saw may have been shared with the flight attendants. I think it’s possible that the hijackers saw the door open & because they were sat closest they were able to rush the cockpit. But said, why would they have stabbed 2 flight attendants if that was the case...

I overthink this because it’s something I can’t make my mind up about fully.

Would appreciate thoughts & opinions.

Thank you