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I want to know, what are all the connections between characters in Tsubasa and the rest of CLAMPS work. Also if there is one central point of all their work, like for instance Clow Reed. I seem to have found a lot Clow Reed being mentioned in some of the works outside of Tsubasa, take for instance that he is responsible for making the Clow cards.

I'm pretty sure you should only see other characters from different stories in either Tsubasa or xxxHolic.
Tsubasa and xxxHolic are connected together. Tsubasa follows Syaoran and the gang, while Holic follows Yuuko and Watanuki, but sometimes they intersect. For example, Watanuki in Holic has dreams with Sakura in them because she is trapped in the dream world in Tsubasa.
Clow Reed comes from Cardcaptor Sakura. If you watch Cardcaptor Sakura, nobody will reference his friendship/rivalry? with Yuuko, because Tsubasa/Holic didn't exist yet, and that was not in his original character design. He was originally only meant for CCS alone.
Then, in Holic and Tsubasa, Yuuko says that Clow is much like her. He has powers like her and a relationship with her (we still don't know it's context though. she seems to dislike him). So the actual Clow from CCS is the same one that she's talking about. It's one of the exceptions where someone from the actual world we see in their manga actually shows up.