ASCII vs Unicode + UTF-8

In modern times, ASCII is now a subset of UTF-8, not its own scheme. UTF-8 is backwards compatible with ASCII.

Yes, except that UTF-8 is an encoding scheme. Other encoding schemes include UTF-16 (with two different byte orders) and UTF-32. (For some confusion, a UTF-16 scheme is called “Unicode” in Microsoft software.)

And, to be exact, the American National Standard that defines ASCII specifies a collection of characters and their coding as 7-bit quantities, without specifying a particular transfer encoding in terms of bytes. In the past, it was used in different ways, e.g. so that five ASCII characters were packed into one 36-bit storage unit or so that 8-bit bytes used the extra bytes for checking purposes (parity bit) or for transfer control. But nowadays ASCII is used so that one ASCII character is encoded as one 8-bit byte with the first bit set to zero. This is the de facto standard encoding scheme and implied in a large number of specifications, but strictly speaking not part of the ASCII standard.