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It only is my point of view but I would suggest to remove the youtube's inclusions and explain "in line" the few concepts they present.
array of numbers
I'd rather have directly introduce the dimensions like this : So a matrix is simply a 2D array. In fact, talking about a MxN matrix relates to an array of length M in which each item is also an array but this time of length N. Usually, M represents a rows' number and N a columns' number.