It's pretty simple. For Legacy and TMCC, you could buy the Legacy system and attach a wire from the Legacy command base to your layout. Your done in most cases. You use the Legacy handheld remote to control the Lionel loco(s) and the DCS handheld to control the MTH PS2/PS3 locos.
For current Lionel production, all new locos also have Bluetooth. You can download the free app to your tablet or phone and control the Lionel loco using your smart device and the app. No need to buy the Legacy system unless you want the additional features. No cost except the loco.
If you buy a current Lionel loco (they also have LionChief Plus or LionChief Plus 2.0) it also can be operated by the Universal Remote (about $40 from most dealers) if you prefer a physical handheld. Bluetooth and LionChief are the two most reliable and simplest control systems made in my experience, and are inexpensive and give you all the basic functions of the loco. Great value.
So you've got lots of choices and don't need to spring for the Legacy system right off the bat if you buy a current Lionel loco that has Bluetooth and/or LionChief capabilities, as most new ones do. Many older LionChief and LionChief Plus locos don't have Bluetooth but do have the LionChief system, obviously. The presence of these systems is noted on the locomotive box if there is any doubt.