Your understanding would be correct, except when there are billions of revenue on the line. Then "the best minds" of the law profession get to work. For example, you cannot make a generic without (legally) having some sample of the drug going off patent. The drug company simply destroys all the expiring patent version of the drug, patents a minor change, and starts the clock all over again. Many other strategies are detailed in this podcast episode.

[0] https://www.econtalk.org/robin-feldman-on-drug-patents-gener...