Actually there are others who have done Legacy upgrades but the two mentioned are most well known.
Lionel is the only source for Legacy electronics. They are only applicable to DC motored engines. There are Legacy electronic boards that are similar to Electric Railroad components but unlike ERR kits they require you to make your own wiring harnesses which all Legacy upgrades require you to do.
There are no manuals. Best have some background in electronics before trying it yourself.
Pete