I received an IRS notice CP5071. How do I know if this is scam or not and why would the IRS send this to me?

This notice is indeed a legitimate request from the IRS, and it's a standard form used to verify identity for tax refunds

Here’s a Turbo Tax article

https://ttlc.intuit.com/turbotax-support/en-us/help-article/internal-revenue-service/got-5071c-letter-irs/L6zjujOII_US_en_US 

And found the IRS Understanding a CP5071 notice

Understanding your CP5071, CP5071C or CP5071F notice | Internal Revenue Service