Only things I could find about this subject always tell that Ledger has a secure element like SIM cards and credit cards have. Also something about another OS running near the main application. But simply when you cut the power isn't it just a storage device? SIM cards and credit cards are encrypted with a pin. I also think the secure element in Ledger is. So can an advanced adversary pull the secure element off and brute force a 6 digit PIN?
No. Secure elements are designed to be resistant to physical attacks. So you shouldn't be able to do anything physical to the chip that allows you to get data off of it. They are designed to prevent unauthorized access to their data so they also are more than just storage devices.
While you can desolder the chip and attach wires to its pins in order to interact with it physically, the chip specifications are not public and require signing an NDA. The manufacturers make it difficult for the general public to be able to know the functionality and ways to interact with the secure element. They are doing security through obscurity.
Furthermore, secure elements are generally complete systems with their own CPU, RAM, storage, etc. So it isn't just a storage device where you can get the contents. Rather they are tiny computers where you can only interact with them in a very specific way.