How to add an outlet to a 3-way switch?

[Joshuma]

I have a three way switch going to my downstairs but I’d like to add an outlet to the top of the stairs switch if possible. I’d also like to add a light to the bottom of the stairs as well but that is for later if it is even possible. Any help or advice would be appreciated. The part that confuses me the most is the power coming from the light. There are four lines coming in and they connected white to black making the white to the first switch the hot. Is there a better way to do this? Why would they do this?
I added some pictures. The one with the switch is how it was wired and was working properly. Please let me know if you need any additional information. Thanks in advance.