All of your income and all of the tax withheld from your various W-2's are calculated together. Your refund or tax due is based on the difference between your tax liability and the amount of tax withheld. If you are getting a refund it is shown on line 35a of your Form 1040. If you owe tax due it is on line 37.
If you had too little tax withheld from your paychecks overall then you will either have a smaller refund or maybe you will owe tax due. That is what happens. If one of your employers is not withholding enough then change your W-4 at that job and have more withheld.
https://ttlc.intuit.com/community/tax-topics/help/what-to-know-about-the-new-2020-irs-form-w-4-emplo...
https://www.irs.gov/pub/irs-pdf/fw4.pdf
https://www.irs.gov/individuals/tax-withholding-estimator
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
