Answer
SQL Server evaluates JOINs using physical operators such as Nested Loops, Merge Join, and Hash Join. INNER, LEFT, RIGHT, and FULL JOINs all use these strategies depending on data size, indexes, and sorting.
INNER JOIN: Returns matching rows only.
LEFT/RIGHT JOIN: Returns matching rows plus NULL for non-matching.
FULL JOIN: Returns all matches + unmatched from both sides.
CROSS JOIN: Cartesian product; typically expensive.
Proper indexing affects which join operator SQL Server chooses.