sorry to ask but what is DMCA?
"Digital Millennium Copyright Act". It's a US law that covers online copyright violations and establishes a process for addressing those. The short version is that it's over 25 years old and was written when the internet was still fairly shiny and new; while it does provide some protection for intermediaries who get caught in the middle (like MangaDex), it didn't really foresee the internet of today and has some problems that should
probably be addressed. It's basically written as 'comply first, and then challenge' as opposed to the other way around, which leads to abuse by less scrupulous actors.
Since much of the internet technically falls under US jurisdiction to some extent (.com, .org, and a bunch of other top-level domains are administered by US based domain-name registrars), it's a law that has some reach beyond US borders.