Good points,
@di anna.
IMO, the killer did what they could:
(1) Electronically to surveil the house and/or any or all of the 4 victims and 2 housemates who were spared in the hours before and around the time of entering the house to kill to scope out the scene:
e.g., possibly they:
(a) watched the Grub Truck livestream showing when Kaylee & Maddie were still out versus heading home,
(b) hacked into neighborhood ring cameras/Alexa apps or secretly installed ring or doorbell or game/deer/wildlife cams behind or next to or in front of the house to watch who came and went and who was up or in bed,
(c) hacked DD software or ordering or general knowledge or acted as an imposter at DD or at the Jack In The Box or had access to hacked info at a minimum to know about the order DD delivered to Xana just before the murder which according to the PCA was right before she was active on SM, which could also have been hacked and/or the killer could have even been messaging or cyberstalking Xana on SM and knew around 4 am she was on her way to sleep, maybe Ethan was already asleep, or at least she was likely super tired at 4 am,
(d) dropped a tracking device in their vehicles or bags/backpacks or used an app to track the victims' whereabouts, and knew when they were all in the house, including DM and BF,
(e) watched them on hacked cameras or cyberstalked them on SM account reading posts from parties, bars, football games, etc. on their activities
(f) had passwords allowing access to their home wifi, and could spy on them that way through Alexa/fitbits/DD orders/monitors, etc. that were connected to the wifi if hackable and not using VPN connections for
(2) Then they used the tried and true old fashioned techniques to confirm what they may know electronically before entering the house:
e.g., possibly they:
(a) looked into the windows with binoculars from far enough away and in a remote portion of the neighborhood that they knew didn't have any video cameras and even saw them turning out lights and going to their rooms and shutting the doors and turning off lights,
(b) watched them come home or knew the timing of when they would be home from electronic means -- for instance, when Kaylee and Maddie left the Grub Truck and got in a rideshare they would be assumed to be home within so many minutes, or Ethan/Xana had posted on SM what a great party, see you tomorrow for brunch or whatever
(c) knew because of how late it was and the general revelry in the neighborhood had died down that it was not a night when they were partying at home that when they came home they were alone at home and on the way to bed, possibly with the exception of Kaylee trying to call her ex bf and then Maddie, whose calls went unanswered because he was asleep at 4 am, but then they probably gace up and went to bed.
IMO, it wasn't that hard to use a variety of surveillance techniques (some possibilities given above that of course may not apply to all 6 people in the house or 4 victims) but could be cobbled together in such as way as to be assured that the coast was clear and they had their window before it got light out at sunrise approximately 2 1/2 hours later to go in in stealth mode in the darkest hour before the dawn and commit their murders. And no one was waiting there to shoot an intruder with their gun, especially if the slider or front door were unlocked and/or they had the passcode to the door locks.
All MOO