The 'What's Mine Is Yours' manga is the origin. Adaptations try to stay true to its core, but sometimes they add or subtract details to make it more appealing or suitable for the specific format. It depends on various factors like the target audience and the creative team's vision.
The mine was a mysterious place that was said to hide many secrets and treasures. The details of this place have not been described in detail, but people speculate that it may be a place full of danger and adventure, full of mysteries and dangerous creatures.
In most cases, it's not a good idea. Adding someone else's story without permission violates their rights and can get you into legal trouble. Also, it doesn't show your own creativity and originality.