For a backdoor Roth to work the total 2020 December 31 combined value of all Traditional,  SEP and SIMPLE IRA accounts must be zero.  If you have a 401(k) rollover IRA and roll it into a 401(k) (assuming your 401(k) plan allows that) then it will not enter into the calculations.

Yes, you can make a 2019  non-delectable contribution  before July 15 and report it on a 2019 8606 form with your 2019 tax return.   You can also make a 2020 Traditional IRA contribution that you would report on your 2020 8606 next year.

Any or all can be converted to a Roth IRA before the end of 2020.   Most people convert immediately to avoid having to deal with earnings within the Traditional IRA.

@sydneysamantha 
@macuser_22 
I am filing amendment for 2019. I am just double checking to see if there are any ways I could do rollover from T-IRA to R-IRA for "2019 tax year" at this time in Oct 2020 ? During amendment preparation, I realised I could have taken advantage of rollover but didn't do it.  

Sorry but you are way past the deadline. 

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@amit2020 wrote:

@sydneysamantha 
@macuser_22 
I am filing amendment for 2019. I am just double checking to see if there are any ways I could do rollover from T-IRA to R-IRA for "2019 tax year" at this time in Oct 2020 ? During amendment preparation, I realised I could have taken advantage of rollover but didn't do it.  

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That would have had to have been completed before Dec 31, 2019.