If I'm understanding you right, you wanna know why we use 'Eso / Esa' instead of 'Esto / Esta'.
The answer is very simple, if you are talking about something you are interacting with, something which is close to you, or is happening right now, then you say 'esto / esta'. If whatever you are talking about, is not close to you, you are not interacting with it, or is not happening right now, then you say 'eso / esa'.
The closest english equivalent is this and that. (This for 'Eso / Esa', and That for 'Esto / Esta').
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