I see answers but they don't make sense to me right now

Yes, form 760C is the Virginia form that calculates underpayment of estimated taxes.

Line 3 on form 760C on your 2023 return is "2022 Income Tax Liability After Spouse Tax Adjustment and Tax Credits". This is used to help calculate if you even owe a penalty for underpayment.

You can get a copy of your 2023 return from the state, but it will be by mail, and may not come for up to 10 days (their estimate). To get a copy, please see this Virginia page.

Note the state of Virginia says this:

Filing Extensions

Can't file by the deadline? Virginia allows an automatic 6-month extension to file your return (Nov. 1 for most filers). No application is required. You still need to pay any taxes owed on time to avoid additional penalties and interest. Make an extension payment.

Please the page above if you have questions on this. Yes, you can file your federal return (1040) now if you want, and your state return later.