Does IOS 15 supports for iPhone SE 1st generation

It should. It's on the list.

iPhone models compatible with iOS 15.4 - Apple Support

This guide helps you get started using iPhone and discover all the amazing things it can do with iOS 15.4, which is compatible with the following models:

iPhone 6s

iPhone 6s Plus

iPhone 7

iPhone 7 Plus

iPhone 8

iPhone 8 Plus

iPhone SE (1st generation)

iPhone SE (2nd generation)

iPhone SE (3rd generation)

How do I update my 1st and 2nd generation Iphone SE to ios 15? If I purchase the SE2022, how many years will Apple support the updates?

Start here: Update your iPhone, iPad, or iPod touch - Apple Support

No one here would know how long Apple will support phones with an iOS update, but the iPhone 6s was able to update to 15.4.1.