1. If this is low-voltage wiring, where is the transformer?

2. if the boxes are inaccessible, will it be a problem when the LED lighting fails and needs replacement?

The transformer is in the light and a replacement light can easily be installed. I am concerned that the j box has 120v spliced at that point. The wire then going to the light and thus the gransformer.

If the hidden box has 120v wires in it, then it isn't 'low voltage' and therefore can't be buried.

If it's truly low voltage, then it can be buried but it's still not usually a good idea (but that's for convenience, not safety).

Thank you !!!

Further, I'd avoid making transformers inaccessible even if it were legal. I had a cabinet installer do that and when the tranformer failed it was a royal pain to fix (I had to remove the cabinet. I guess one can argue if removing the cabinet to get to the box met the code requirement of accessibility, but it was still a pain in the rear.