I have always wondered big prjects like Tamriel rebuilt or the province mods worked, I assume they have multiple people working on them so how does that work?

Big projects like TR and Province Cyrodiil do indeed utilize the collective efforts of (sometimes very large) teams of modders. TR in particular has had probably over a hundred people work on it over the past 13 years. It takes a lot of organization, sometimes a team member's sole job is to simply make sure other people do their jobs without messing things up. You can get a lot more information from listening to Darkelfguy's interviews with a TR team member and a Province Cyrodiil member, or by visiting their websites.

i can definitely tell you you need planning and a precise idea what you're gonna do.

i can tell so because mostly everything even somewhat biggish i've ever done totally excessed and escalated due to my lack of doing so

Why not head over to TR's website?
They have a http://tamriel-rebuilt.org/?q=content/frequently-asked-questions and a http://tamriel-rebuilt.org/forum/ that shows how the mod is being built.

Big projects take a lot of planning, time, people, and commitment.