can I find 2021 forms on the turbotax site?

You cannot prepare them "simultaneously."

Each tax year has to be filed separately using the forms  and software for the specific tax year.  They cannot be combined in any way--do not even put them in the same envelope when you mail them.  Software for past years is available back to 2021.  Any earlier years can only be prepared on paper forms by hand--and those refunds have been forfeited now.

The current online program can only be used for a 2024 return--no other year.  And if you did not file a timely 2023 return you will have trouble e-filing 2024.  When it asks for your 2023 AGI try using zero.  If zero does not work then you have to print, sign and mail 2024.

It might be better for you to seek local paid tax help in a situation when you are several years behind in filing your tax returns.  Do you even have all of the documents you will need to enter and show your income—like your W-2’s and any 1099’s?

You cannot change the tax year.   The current online program is for 2024 only.   Only a 2024 return can be prepared online and only a 2024 return can be e-filed.

Online preparation and e-filing for 2021, 2022, and  2023 is permanently closed.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.

To file a return for a prior tax year

 If you need to prepare a return for 2021, 2022, or 2023  you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.

Remember to prepare your state return as well—if you live in a state that has a state income tax.

https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/00/25573 

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.