First of all, it depends on whether the contract between designer A and company A has clear provisions on intellectual property rights, that is, provisions on the time and geographical scope of exclusive use rights.
If there is no explicit agreement, then Designer A has the copyright, Company A has the right to use it, and Designer A can authorize a third party to use it.
If there is an explicit agreement, the rights will be exercised in accordance with the agreement.
“Company B applied for a packaging patent in 2018, and the designer is b.” Are the products designed by designer A and designer B exactly the same? Or similar?
If a package is protected by a patent, it depends on whether the patent is stable. If designer A can prove that the packaging has been used publicly before the patent application date, then Company B’s patent may be unstable. Likely to be invalidated due to lack of novelty. If it cannot be proved, Company C’s use may infringe the patent right.