First, we need to discuss a couple of topics.
TLDR
First of all, the law is complex and tricky. It requires years of focus to learn enough about it to become tolerable, and it varies across countries, juristictions, and districts.
The law does not make things legal. The law makes things illegal. Judges and juries don't detemine if an act is legal, only if it is illegal. If there is no law or decision making something illegal, then an argument can be made that a not-illegal act is therefore legal. (See also other legal definitions, like unlawful.)
NOTE: Legal, illegal, and unlawful all mean different things.
In the U.S., if the legislature passes a law with ambiguous language, it can be elevated up to the courts to "interpret" it. What is actually law is not always the written word of the law, but rather a particular judge's interpretation of that law, written down in a decision.
Unless you are a lawyer who specializes in copyright law, you are not a lawyer and cannot reliably discern how the law applies. This includes myself. I am not a lawyer. Anything I write here is meant to be informational, but is not meant to be construed as legal advice.
Having said that, I co-founded an internet startup from 2006–2010 where I had to dig into copyright, fair use doctrine, and the U.S. Digital Millennium Copyright Act of 1998 (DMCA), trademarks/servicemarks, and patents in the United States in order to make sure we stayed in-bounds, legally speaking, while we were building our product. So I am not a lawyer, and this is not legal advice, but I'm also not talking out my ass.
Despite the rhetoric, most countries do not have a "Justice System"; merely a "Legal System".
TLDR
"No copyright intended."
If you've ever seen this in the description of a YouTube video, it means that the poster of that video has absolutely no idea how copyright works. Copyright and Authorship are NOT the same thing. And saying that you "don't intend copyright" (whatever that means) while performing coopyright infringement does not clear a person from responsibility.
-
Authorship refers to the author or creator of a thing. It could be a book, a song, a video, etc. Nobody can take that away from you — if you are the author, then you you are always the author.
-
Copyright refers to the legal right to determine how copies are made. By default, copyright lies with the author. But an author can sell, exchange, or give-away their copyright to another entity (e.g., person, company).
-
Fair Use is a very specific legal construct in the world of copyright. There is specific criteria required in order to achieve a Fair Use exeption in Copyright law. Just because a person claims "fair use" doesn't mean they qualify for it. Sometimes "remixing" falls under this exemption, but only if the result of remixing is sufficiently different enough from the original to qualify as a "new creative work" in the eyes of the law.
Let's take Nintendo as an example — a company that vigorously protects its intellectual property. When Nintendo Switch Online (NSO) launched, some people accused Nintendo of taking the work of ROM hackers and shipping it in their product. And in this entitled world of ours (certainly not limited by age or generation), people started writing "Fuck Nintendo!"
Hypothetically speaking I were to perform the work to dump the SNES cartridge of my favorite game ever, Super Mario World, I would be the author of that ROM dump, but I would not be the copyright holder of that ROM dump. Nintendo is. I would have broken copyright law by bypassing copy-protection, and the ROM dumping process would be making a copy of Nintendo's intellectual property through unlawful means. Even if I were to hack the ROM with 99 lives and permanent invincibility, it would not be different enough from the original to warrant a "new creative work", and therefore, Nintendo would still have the legal right to determine how copies are made, including their own copying of fan-made hacks into ROMs.
TLDR
As it applies to ROMs and BIOS files, these chunks of binary data are the intellectual property of the copyright holders, which are commonly either the Platform Owners (e.g., Atari, Commodore, Sega, Nintendo, Sony, Microsoft) or the Game Publishers (e.g., 2K, EA, Ubisoft, Activision, Bethesda), but not usually Game Developers.
BIOS files are digital dumps of the BIOS of the game hardware. Emulation of earlier game hardware doesn't usually require these files, but as the on-board software became more complex, emulation started requiring dumps of this highly-proprietary software in order to boot the virtual hardware and perform the emulation.
(When we reached the Wii/PS3/Xbox/Xbox360 era of devices, they started to come with legitimate operating systems, running on top of the BIOS. But that's a whole other topic.)
Because BIOS files represent highly-proprietary intellectual property owned by their copyright holders, and those copyright holders have not granted me/you/us a legal license to copy, host, or otherwise make available these files without violating copyright law, I (as a United States citizen) cannot share these files without legal repercussions.
Countries with (a) weaker copyright laws, and/or (b) don't honor the international laws around copyright can share them because the act is either (a) not unlawful in their country, or (b) their legal system won't take action against them.
Now… I said all of that in order to say this:
Most websites will not provide download links to BIOS files for you, even if they have them. Because for most websites, that would be illegal, and legal action would be taken against them. And even if you were to find these files posted somewhere, it does not mean that it is posted there legally.