Rolling Upgrade - Minimize downtime during Migration and Upgrade

Sooner or later it is time to move the Oracle database to a newer version, and often to another server. The main question is always - how long a downtime can be tolerated? It is ideal without downtime, but in practice there are many factors that need to be taken into account in order to minimize the duration of downtime. The more money you put in, the shorter the downtime. Some compromise needs to be found.Listen to my experience from one such real world project, in which I migrated one Oracle database from an on-premises server to the Cloud, and upgraded from the version 11.2.0.4 to 19.3.