I still haven’t received my stimulus check, and I don’t know why. I received my 2020 return via direct deposit. This is my first year filing taxes, could this be why?

You have not yet received the third stimulus because you did not file a tax return for 2019 and the IRS has not yet processed your 2020 tax return. You will receive it after the IRS has processed your 2020 tax return.

The IRS says in this document:

If a taxpayer's payment is less than the full amount and is based on their 2019 return, they may qualify for a supplemental payment after they file their 2020 return. The IRS will automatically reevaluate their eligibility. If the agency determines that they are entitled to a larger payment or the full payment, it will send them a supplemental payment covering the difference between what they originally received and the larger amount. If the re-evaluated amount is smaller, they won't need to pay back the difference. Aside from filing a 2020 tax return, no action is needed on their part.

Hi I appreciate your response! 
When exactly does the IRS process tax returns? I already received my refund, so do I just wait until after the filing deadline? 
I apologize for all the questions.

The IRS will do a sweep of the 2020 tax returns after the filing season. No exact dates have been given by the IRS, but you should not expect it before July - August this year.