Part 1: How can I upload Universal Analytics data to GA4?

It may have come across your mind to use Data Import Feature in GA4 but this isn't an option to upload your UA data to GA4. Data Import feature allows you to upload data from external sources which you can then join with your existing GA4 data. This means that you are unable to upload historical data that was captured way before your GA4 was setup. Additionally, the types of data that you can import to GA4 is limited to offline events, cost data, user data and item data.