How do plants decrease the local entropy and still obey the Second Law of Thermodynamics?

As a result of photosynthesis, plants take randomly arranged (high entropy) molecules (mainly water and carbon dioxide) and convert them to ordered (low entropy) chemicals (in the plant cells).

So there is local entropy-reduction (of the water and carbon dioxide molecules) when the plants grow

However, the energy from the light (used during photosynthesis) is converted to heat, heating the environment during the metabolic processes. This increases the entropy of the enviroment. This effect is bigger than the entropy-reduction.

Therefore the overall effect is that there is an increase in entropy.

It's not closed. We get energy from all kinds of things, the sun, the earth's spin, the earth's core. Energy keeps entropy in check and sometimes makes it look like it lowers entropy.

They are not a CLOSED system. They exchange energy with their environment.

life is a temporary exception to the laws of thermodynamics.