As of Windows NT 6.x, Windows OS cannot be installed on FAT or FAT32 partitions, as Windows use extended functions of the NTFS file system such as hard links, symbolic links and junctions this is not possible on FAT!
1. Does Windows 11 have to be installed on C:?
No! You can install it on all partitions, when starting Windows-11 this will be "C:"
2. How big should the SSD / HD be?
Windows 11 requires 64 GB but it can also be more!
3. Do I have to format my PC to install Windows 11? No! You can install it on the existing partitions over the old operating system, with at least 64 GB (better 128 GB) and NTFS file system.
4. Can the FAT file system be read by Windows-11?
Yes FAT, FAT32, ...! But installation must be done on NTFS!
(Image-1) The Windows 11 file system is NTFS and not FAT!