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Than you!
I don't know exactly how we measure the mass, I have not studied anything related to that yet, but I guess that if we take a look at how the light is curved due to the gravitational field of the black hole, we can estimate the mass of the black hole through the relations that General Relativity establishes.
There must exist other ways, but I don't know them.
Regards friend, and thank you :)

Yeah, that makes sense. Thanks!