How do I sew a ripped crotch on a pair of jeans?

Patch it.

https://youtu.be/h90nYzbDFdE

https://youtu.be/w3uYH3uIFqY

https://m.wikihow.com/Fix-the-Crotch-Hole-in-Your-Jeans

If they have split on both sides I maybe time to replace them. Sewing A demin patch in the crotch area can be quit uncomfortable and unsightly.

How is the crotch seam ripped? Are just the stitches in the seam pulled out, or is the fabric itself ripped off the original seam line?

Denim is a difficult fabric to work with, as it frays very easily. As a crotch seam takes a great deal of strain, it is important that there be adequate seam allowance left in order for you to be able to stitch the seam back up, and so that it holds together when wearing the jeans again.

If there is insufficient seam allowance on either sides of the seam, then you will have to patch it with an additional piece of fabric.

It sounds like your jeans have seen their better day. Maybe it's time to replace them, but you don't have to ditch them altogether - you can repurpose them into some fabulous projects:

https://www.hometalk.com/diy/decorate/pillows-throws/cute-padded-denim-hearts-made-from-your-old-jeans-27054390

https://www.hometalk.com/diy/decorate/rooms/quick-easy-hand-sewn-upcycled-denim-storage-basket-7461378

https://www.hometalk.com/diy/decorate/pillows-throws/gorgeous-boho-style-recycled-denim-pillows-31468498