How to make estimated tax payments

Hello, I did an estimate of taxes owed for 2020 and think that the taxes owed may be more than the taxes that I have withheld through salaries and pension.  I have not used the quarterly estimated payment method during the year since the taxes so far are withheld through salaries and pension distributions.  My questions are:

1.  Can I send an estimated payment and indicate it as the 4th quarter estimated tax payment to cover the difference?  Or do I just send it as an estimated payment and do not need to indicate which quarter (since I have not used the estimated payment method during the year)?

2.  What is the best way to send in the payment?  I read the 1040 estimated payment instructions, which contains four quarterly coupons, so I can use the coupon for the 4th quarter to make the payment?  Or I can log into the IRS web site to make a direct payment from my bank account?

3.  If I send a check with the 4th quarter coupon, would I get some kind of receipt to show the amount that I pay as estimated tax, which I can add to my taxes withheld from salaries and pension?

4.  If I make an estimated payment online, what would the IRS website payment screen show?  Would it show I am making a payment as the fourth quarter payment, or just a payment?  Also, would I get some kind of receipt from the web site payment screen as record/evidence to show that I made a payment and the amount to add to my other tax withheld?

5.  Also I think the 1040 or 1040SR has a separate line that I can write in the estimated payment, and this line is added to the tax withheld line to derive the total taxes paid.  Is it correct?

5.  How secure is the online payment method?

Thank you for your help in advance.