If I bought a used car end of 2020 and immediately started using it for business...would i do half year convention (MACRS) or mid-quarter convention? Car cost $58k

Turbotax should automatically compute the proper depreciation and use the proper convention if you entered the info about it correctly. including the date placed into service. if your trying to do the 2020 return, online preparation is closed for 2020 you need  a desktop version

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

If this was the ONLY asset placed in service for the year then MQ would be correct.