merge lists of lists

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I realize that if I have two lists, say

l1 = [1,2,3]
l2 = [4,5,6]

then I can join them via

l1 + l2

i.e. this gives [1,2,3,4,5,6].

I want to know how to do this when my list is not a list of numbers, but a list of matrices. I imagine it would be the same for a list of lists but I don't know how to do that either.