Examples:
1. Mike is in 'general' (channel name) channel. Go there so you can chat with him.
2. Mike is on 'general' channel. Go there so you can chat with him.
I would say "on" because he is not physically inside it, but I have seen native English speakers saying "in". How come?
Channel can be used with either preposition, depending on the context. The ferry boat is in (not on ) the channel. Sesame Street comes on (not in ) channel 8 where I live.
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