my guild has this information available in discord. Its pretty transparent if you are involved in the guild.
I’ve always seen it in huge piles in the Guild Bank. Usually free to grab but there’s so much it’s still overflowing.
That’s a good question. All the gold is directly deposited to a bank alt from there the bank alt invests it into the auction house where a large profit margin can be made AND ITS GONE! 
May I join? 
My old guild was making bank. The MT had everything he wanted to (friends with the GM), but nobody else (offtanks, me as MT of Raid Team 2) could get anything. I got to Honored with Thorium Brotherhood on my own, but they wouldn’t give me cores for the FR pieces (“we’ll talk about it later and see what pieces make sense…”).
They take it to Morodor and destroy it in the fires of Amon Amarth.
But are horde, you are the enemy
Depends- good guilds put it towards flasks, consumes and gear for later phases. Decent guilds spend it on a lot of FR now to try to get through MC.
Bad guilds pocket it and use their players like a sponge. That’s just part of Classic- never knowing if your leadership is doing what’s best for your guild, or using you like slaves to farm for them.
It’s why Vanilla was the best xpac ever, so easy to abuse everyone else. Exploiting, ninja looting, botting, stealing guild mats and wasting thousands of hours of other people’s time is how you play this game effectively.
with my guild
most gold obtained from BOEs goes towards arcane crystals, black lotus, and materials to craft things like hide of the wild for the raid.