Learn Hardcore Chinese: Zài péngyou zhōng, dàjiā dōu juéde zìyóu biǎodá zìjǐ de xiǎngfǎ hěn zhòngyào. - Among friends, everyone feels that being free to express their own ideas is very important.

Breakdown of Zài péngyou zhōng, dàjiā dōu juéde zìyóu biǎodá zìjǐ de xiǎngfǎ hěn zhòngyào.

In the phrase 在朋友中, what does 中 mean here? Can I say 在朋友里 instead?

在朋友中 literally means "among (my) friends."

So 在朋友中 = "within the group of friends" → "among friends."

You can say 在朋友里, but:

在朋友中 or 在朋友当中 is more idiomatic for "among friends (as a group / in that circle)."
在……里 usually emphasizes a physical "inside" or a more concrete container (in the room, in the box, in the book), though it can also be used more abstractly.

For "among friends," 在朋友中 or 在朋友当中 is the most natural choice.

Why do we have both 大家 and 都? Aren’t they both “all”? Isn’t that redundant?

They do both relate to "all," but they function differently and often appear together:

大家 = everybody / all of us (subject: the group of people)
都 = all / both (adverb: emphasizes that the statement applies to every member of the group)

Structure here:
大家（subject） + 都（adverb） + 觉得……

So it’s like saying:
"Everybody all feels that..." → "Everybody feels that..."

In natural Chinese, 大家都 is extremely common and doesn’t feel redundant.
You could say only: