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Biased-ness of OLS estimates

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Hey guys, so I'm just reading through the proof of the biased-ness of OLS estimates if an important independent variable is left out and I'm just stuck at the very end, basically the mathematical part that confuses me is the part circled in red.

When we take the expectation of how does it simply end as the same thing?

Ie, Why is ?

Since isn't a constant as it depends on so it's like a random variable which outputs different values as changes, so why does it follow the rule that where is a constant?

Thanks :)