Bitmap storage / memory representation for N-bit RGB bitmaps

Bitmap storage / memory representation for N-bit RGB bitmaps

Typically, the most common RGB format seems to be 24-bit RGB (8-bits for
each channel). However, historically, RGB has been represented in many
other formats, including 3-bit RGB (1-bit per channel), 6-bit RGB (2-bits
per channel), 9-bit RGB (3-bits per channel), etc.
When an N-bit RGB file has a value of N that is not a multiple of 8, how
are these bitmaps typically represented in memory? For example, if we have
6-bit RGB, it means each pixel is 6 bits, and thus each pixel is not
directly addressable by a modern computer without using bitwise
operations.
So, is it common practice to simply covert N-bit RGB files into bitmaps
where each pixel is of an addressable size (e.g. convert 6-bit to 8-bit)?
Or is it more common to simply use bitwise operations to manipulate
bitmaps where the pixel size is not addressable?
And what about disk storage? How is, say, a 6-bit RGB file stored on disk,
when the last byte of the bitmap may not even contain a full pixel?