This is essentially a reference request. According to the classification of the finite simple groups, there are 26 (or arguably 27) sporadic groups. Let us denote these by G_1 , ... , G_{26} (resp. G_{27}).
Question: Which G_i can be embedded into which G_j?
I was unable to find the answer to this question in the literature, but I suspect that the experts will know where to look. Note that it is not too difficult to find a diagram describing the weaker property of "which G_i is a subquotient of which G_j."