Please see this link.
(hope it doesn't rot)
Is this proof circular? I think similarity is proved by basic laws of trigonometry, especially Pythagoras theorem. When I search for proof of Similarity, I'm getting proofs using similarity. I have heard this proof is also written in the famous book "$367 $ Proof of Pythagoras Theorem". There is nothing much of my attempts to be shown, as this question is driven by intuition.
No, the proof is not circular. Since your question is regarding the method of proof and not the proof itself, I will not elaborate on the proof but rather I will point you to Book VI Proposition 4 from Euclid's Elements for a 'proof of similarity'. Propositions 5, 8, and some others in that book are also relevant.
Similarity, far from being proven by trigonometry, is the reason why we can reliably do trigonometry and much of elementary geometry in the first place.