The 1040SR is EXACTLY the same as the regular 1040 just larger type so it goes on 3 pages with the Standard Deduction chart on page 4. But it has the same line numbers. You automatically get the 1040SR if you or your spouse is 65 or older. It doesn't matter which one you file.
See the instructions are the same for both the 1040 (and 1040-SR)
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
In the Online version it will show the regular 1040 on the screen but if you print it out or save the pdf it will be the 1040-SR.
If you are using the Desktop program and look at it in Forms Mode it shows the 1040 but if you print it out it will be the 1040SR. There is a way to force the one you want. You can turn off the 1040 SR and save it as the regular 1040 form. Switch to Forms Mode and open the Info Wks. Scroll down to Part II Address and Federal Filing Status. There is Print Form 1040-SR instead of Form 1040 - Yes or No.
If you really want to use the regular 1040 in the Online version, you can print your return and copy the amounts into a blank 1040 (link below) from the 1040SR. But then you will have to print and mail it.
https://www.irs.gov/pub/irs-pdf/f1040.pdf