Different versions of the client have different supports for different codecs. They're usually fine with h264 and h265 but newer (av1) and older (avi/divx) are supported sporadically. I know the Android client only does up to h265 on Amazon devices and that's generally what my family is using to access the library. I try to avoid transcoding as much as possible. It's not worth it to have a tiny file size if you're just going to be needing to reprocess it to be viewed on most of your clients. The desktop client typically has the widest range of available codecs but I don't know if it works with AV1 yet.