Biped files are be used since almost the beggining of 3ds max. Now days is pretty much one of the standarts animation files for mocap and keyframe animation. Also the same .bip file can be used on any version of 3ds max. Is been used by big studios like Ubisoft, Blizzard and more other companies till today. Please note that this site provides .bip files from version 2011+

Biped has a pretty unique stracture. Is been the same for a lot of years and has almost zero errors when importing the biped skeleton into other programs, like for example Unity and Unreal engine.

Yes but not directly. best option is to export them as fbx file. All other programs like Motion builder, iclone, Unity, Unreal, etc has automatic retarget of biped animation with almost 1 click.

Is a mix. Some are build from scratch but most is mocap that is been "cleaned" and is ready to be used on any project with minor adjustements if needed. Such us using biped layers to adjust hands head or feet if the bip file is not matching that match with the stracture of the character that is been loaded. But in most cases they files work without any adjustment.