Question

how are insects digested by plants

Answer 1

Some insects are digested by plants when they are dead. They digest them in this way:
When dead insects fall dead into the soil, the roots of the plants take their nutrition and digest them...
There is another way:
Some plants like Nepenthes, etc. (insectivorous plants that grow in places deficient in nitrogen) digest the insects to fulfil their nitrogen needs.......
Hope this helped.........

Answer 2

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The carnivorous plants use enzymes like the ones in your digestive system to break down their prey. Then they absorb the nutrients (such as nitrogen) and use them to make their own proteins. Some of these proteins will be the digestive enzymes that help to dissolve the next unlucky insect.