Why Le thé and not just thé. Doesn't le thé translate to the tea?
When making a general statement about a subject, you need the French definite article.
Nouns in French do not usually stand alone. You cannot have just "thé" in a sentence without modifying it with something.
Is "du thé" also correct?
No, it is not. When making a general statement about a subject, you need the definite article.
Why is the tea masculin but the feminine version of a "une" is used?
Because "une boisson" is feminine. Each noun has its own gender.
Is "un thé est une boisson" also correct in this case?