UEFI/Legacy BIOS Graphics Card support?

1a) Graphics card vendor websites have BIOS support information- Legacy/UEFI/both. What does that actually mean? I think I know what UEFI and Legacy BIOS are. Does the graphics card also depend on which type of bios I have? Is this really important? If its important then why Evga or Asus website didn't mention BIOS support?

1b) Why Some GPU supports only UEFI, some only Legacy and some both? for example- sapphire r9 nano only supports legacy bios according to website. r9 fury x supports only legacy and fury only supports uefi bios

1c) Is there any backward compatibility issue here?

1d) If I have a old legacy BIOS p55 with a core i7 or AMD old motherboard with Phenom II x6, will it support sapphire r9 nano(legacy) and r9 fury (uefi)?

2a) If a motherboard has UEFI BIOS(and has a pciE16x) will it be able to run any graphics card which has UEFI support? same question goes for legacy.

2b) If a GPU says that it supports both UEFI and Legacy will it run properly on any motherboard (with x16 slot)?

2c) Does this support ability also depend on operating system? If so then let me know some details about windows 7/8/10 graphics card support.

3) r9 280x sapphire or many other radeon device has a dual BIOS switch, what does it do, does it have to do anything with UEFI/Legacy support? Or is only for overclocking? In which mode should I set it?

4) Does this UEFI/Legacy BIOS support depend on the chip makers- AMD and Nvidia or does this support actually depend on the vendors? I mean if chip maker restricts a GPU to support only UEFI can the vendor, will Evga or Sapphire be able to unrestrict it?

5) Is EFI bios is exactly same thing as UEFI? I saw some motherboard vendors (Gigabyte) mentioning EFI bios not UEFI.

6) Does this UEFI/Legacy only important for GPU? or is it crucial to check this support when buying SSD or other components as well?

7) What is Asus AMI bios? that's Legacy bios right?