As the name suggests, a "shoving platform" is a car that would be placed at the "front" end of a shoving move to give the brakeman/trainman a safe place to ride, vs. hanging off the side ladder of a car.
There's not much else to google on the topic as the full extent of how it's used is "a place for the trainman to ride when shoving a cut of cars".
When NOT making a shoving move, it's just another piece of equipment to haul around so it wouldn't matter where it ended up in relation to the rest of the consist. So depending on what switching the crew is doing it'll either just end up wherever is most convenient, or placed so that it's in the right place when needed. Depending on the train's operating pattern, and where the shoving platform is actually needed, there could very well be much of the run where it's beside the engines, where they can do runarounds on the main part of their route, but there's that one spur where a long shove is needed.
For a really simple example, let's say I'm operating a local train that runs out of the yard at A let's say about 10-20 miles to B, where there's a runaround on the main track, and a facing point (meaning I have to run around at B to shove into it) spur that requires a platform due to length or other safety concerns. In the yard at A, I'd build my train with the shoving platform (SP) next to the engines. Run A to B. Runaround the train and shove down the spur SP first. Complete switching. Run back to A with the SP on the rear. If I left A with the SP on the rear, it'd be at the wrong end when I got to B, and I'd have to use the runaround track to move it to the other end. Six to eight extra moves just to move that one car.
If this was the caboose era, yeah, that's just the natural moves you'd make to keep the caboose on the rear. But for a shoving platform, you only need it for the shove move, so you marshall the SP so it's in the right place for the shove and you don't have to juggle it to the other end at B. It doesn't need to be on the rear of the train any other time.
The other potential curveball is that a few railroads have also converted old cabooses to remote control/robot cars, which would be coupled and electrically connected to the power in order to make any engine a belt-pack remote-controlled unit without having R/C equipment installed in the engine itself.