Learn Spanish (Spain): Lo extraño es que hay alguien aquí. - The strange thing is that there is someone here.

In Spanish, you can put the neuter article lo in front of a masculine singular adjective like extraño (strange) to turn it into an abstract noun. It translates into English as "the strange thing", "that which is strange", or "what is strange".

Could I say el extraño instead of lo extraño here?

No, because el extraño would refer to a specific masculine noun, usually meaning "the stranger" (as in, an unfamiliar man). Using lo keeps it abstract, referring to the general concept or situation of strangeness.

Why do we use hay here instead of a form of estar?

Hay means "there is" or "there are" and is used to state that something exists. You would use estar (like está) to say where a specific, known subject is located. Since we are just pointing out the existence of an unknown "someone" in this place, hay is the right choice.

Does the word alguien change depending on who is here?

No, alguien (someone / somebody) is an invariable pronoun. It always stays exactly the same, regardless of whether the person you are referring to is male or female.