Learn Russian: Это горячий чай, а это холодный сок.

This is because of a very important spelling rule in Russian! The standard masculine ending for adjectives is -ый, which you see in холодный. However, after the letter ч (and six other specific letters: г, к, х, ж, ш, щ), Russian spelling rules forbid writing the letter ы. You must use и instead. That's why it is spelled горячий.

Since чай and сок are masculine, why do we use the neuter word это instead of the masculine этот?

Because here это acts as a fixed phrase meaning "This is...". Whenever you are introducing or pointing to what something is ("This is hot tea"), you always use the invariable word это, regardless of the noun's gender. You would only use the masculine этот if it was attached directly to the noun, like in этот чай горячий (this tea is hot).

Why is the word а translated as "and" here? Shouldn't "and" be и?

Russian has two main words that translate to "and": и and а. You use и when simply joining similar things together. You use а to show a contrast, a comparison, or a shift in focus—often translating as "but", "whereas", or "and on the other hand". Because this sentence contrasts hot tea with cold juice, а is the natural choice.