https://wikimedia.org/api/rest_v1/media/math/render/svg/6c66...
Does the sum include both terms or only the first? I happen to know what Covariance is, but if I didn't I wouldn't be clear. I'd probably guess right, but scroll down a bit to this:
https://wikimedia.org/api/rest_v1/media/math/render/svg/e634...
Is the +2 a multiplier on the second term or additive within the first sum? I try and read it from the spacing but I honestly can't tell.
BUT, for your specific examples (in case that is an actual question):
The first one can be parsed using the fact that y_i in the second term wouldn't make sense if it was outside the summation, since the i index is only defined inside
The second (and actually, the first one, too) uses the very well established convention that multiplication always has precedence over summation, in every kind of expression. So the 2 multiplies the second sigma, and then the product is summed to the first sigma. This rule should make a lot of cases much clearer.