A collection holds several fonts, which web pages and many tools cannot use as one file. TTF installs on every operating system and works in every design tool. TTC is at home in font families that share outlines, such as CJK families; TTF is the usual choice for installing a font on a computer or in an app.
One TTC can produce several TTF files (one file per font); when it does, they are offered together as a ZIP download.
The outlines and tables of the font are copied unchanged and only the wrapper around them changes, so the letterforms, hinting and layout features are exactly the same. A collection holds several fonts that share data; each one is written as its own TTF, and several come as a ZIP. A file that is not a TrueType, OpenType, WOFF, WOFF2 or collection font is refused.