Why Is Begonia Dying

Question

begonia leaves are dying, something eating inside of plant in the stem – brown and hole

Answer 1

It's hard to tell without seeing the plant, but I'm guessing it's suffering from pythium root and/or stem rot. You may need to sterilize your plant's soil. Here's a helpful article:
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonia-root-knot-nematodes.htm