That trick won't work. An extension cord is only seen as such as long as it is not permanently attached to a structure, and definitely not when it powers a fixed structure. In other words, it needs to be temporary and to be disconnected when not in use. As soon as you connect it permanently affix it or bury it you are violating the electrical code.

Otherwise this would act as a giant loophole around the electrical code, you could simply use a high current plug to connect your distribution panel and call it a day.

https://plasticinehouse.com/can-i-use-an-extension-cord-to-p...

I'm pretty sure it's against code in CA to install an extension cord permanently inside a structure or conduit like that.

When I put all the cables in the wall for my TV, I ran Romex from the bottom junction box to the top junction box, despite the bottom junction box having a male electrical plug. An extension cord plugs into that from another outlet. (Ideally I would have just hardwired the upper outlet, but that would have been another can of worms.)

Not a good idea. The outer layer is plastic, underneath that something that is best described as an oversized shower hose. Burying it will allow you to take vehicles, wheel barrows, lawnmowers etc over it without damaging it.

I bought a ton of this stuff surplus from a dam construction, that served me quite well in Canada, some of the outbuildings were quite far away from the main house. It's pretty easy to work with, though stripping it is a bit of an art (you really don't want to damage the inner wires).