I have tomato plants that the bottom leaves turned yellow

Question

Also, the small tomatoes are turning rotten on the vine.

Answer 1

There are a number of reasons for leaf yellowing in tomatoes. This article should help with pinpointing and correcting the cause of yours: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm

The rotting is called blossom end rot. This article will help you with that:
https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm