It sounds like the new owners bought the stock of the company, like they bought IBM stock. The company includes its assets, of course.

That would NOT be an asset sale. An asset sale would be the assets(equipment, customer list, etc.) but not the company itself. The company sells the assets out from the company.

In a stock sale, the company still exists and there is only one tax return, 1/1/20-12/31/20. The P&L gets allocated out by the ownership pcts. for the periods of ownership.

In an asset sale, the old company could be liquidated and a short year for the final tax year, and the new owners have a new company for a short year return for the last few months of 2020.