Does this have to be configured manually?

Yes, you can configure it pretty easily, and then reload using the command

Can you make the grass drop the same as vanilla grass without the need for configuration?

https://juraj-hrivnak.github.io/Greenery/drops.html

Use the seeds keyword

Is it the same drop as vanilla grass by default?

"seeds | 0.2"

Regarding aquatic plants, do I also need to set them to spawn in Simple Difficulty Underdog saltwater?

Is it the same drop as vanilla grass by default?

and why doesn't it drop plant fibers?

Yes, you need to add the fluid to the configs

Pyrotech doesn’t use Vanilla drops but Dropt.

Can any compatibility be added?

how did you configure this in underdog?