Results about the existence of continuous functions in topology

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Given a topological space, what theorems or results are there that shows the existence of continuous function between that space to another space, or between some subset to an open set of the same space. For example, we have Invariance of Domain Theorem by Brouwer that indirectly shows existence of continuous injective function by verifying the subset to be open set. I am curious if you could suggest me similar results like it, regardless of topology or not.