I'm on MacOS. I downloaded my prior returns but can't save them since there's no application to open them. How do I solve this?

for each year you would need the desktop app. only the years 2018 and forward are available.  if you paid to file each year, you should contact support to see if they'll provide a download free.

5am-5pm Pacific Time (8-8 Eastern) Monday - Friday
https://support.turbotax.intuit.com/contact

800-446-8848

note TurboTax (and IRS) only retains like the last 7 years after that they are deleted. for years prior to 2018 if you want readable copies you can use this website which for a fee will provide you with a pdf of the return. can't vouch for them but I have seen no threads on this forum complaining about their service

https://www.taxprinter.com/