If someone is in my best friend list, does this also mean I am in their best friend list as well?
Yes, the software accounts the time you've talked to your buddy and then places you into the Best Friends. This happens when you talk to him for a longer period of time. Once the system detects that you haven't spoke for a certain time, the user will be removed from your Best Friends list.
For additional information use the following link: https://support.snapchat.com/en-US/a/bestfriends