I'm trying to build a VOD service and not able to find a way to decide to which resolutions and bitrate a video should be transcoded after uploading. Like what if someone upload a shoot on a mobile phone.
I'm using AWS mediaconvert for transcoding.
There is no absolute rules here so you may find it easiest to simply look at some similar services to the one you want to offer and examine the resolutions/bit rates they are using.
Many services have ways to allow you look at what rates are available manually or even view how the stream changes over time - e.g.: https://stackoverflow.com/a/42365034/334402
It's worth thinking of your target devices also - if you are mostly targeting mobile phones (a service like Tik Tok for example) then you may not need the largest resolutions, while if you are targeting mainly large screens or VR devices high resolution may be very important for you.
Similarly the expected network connectivity will play a role - if most users will be at home and in a region with good connectivity then you may find less need for lower bit rate streams.