A “20-year whole life” policy usually means you finish paying premiums after 20 years, but the coverage itself stays in force for your entire life.
Once the 20-year payment period ends, you own the policy fully paid up: no more premiums are due, yet the death benefit continues, and the cash value keeps growing (often at a slower, guaranteed rate) for as long as you live.
At that point you typically have a few choices. You can simply keep the paid up policy in place. You can withdraw or borrow against the accumulated cash value. Or you can surrender the policy entirely for its cash surrender value, which ends the coverage.
Some insurers also let you use dividends, if the policy pays them, to buy additional coverage.