Do media containers contain multiple audio channels in separate streams?
Depending on the container format, multi-channel audio can either be in a single stream or in separate mono streams.

Most consumer formats allow both, but it's easier for the end-user to have it in one stream (it can be interleaved PCM audio, or encoded bit-streams as ac3 or aac). Notice that it doesn't prevent to have several multi-channel streams in one container (like several 5.1 mixes, each one being one stream, in ac3 codec in a single container)

The MXF OP1A often used in broadcast usually has several mono streams for multi-channel content. A 5.1 mix will then use 6 mono streams in the container.