The learning phase is the period when the Google algorithm learns how your campaigns are run, how much you spend, and how it could save you on the price per click.
The learning phase will manifest itself in campaigns that have more fundamental changes. One of them is of course the transition to a new CSS partner. Usually, the learning phase starts the day after switching to another CSS service and lasts from 1 to 10 days.
The learning phase occurs when you use a smart bidding strategy, such as Maximize conversion value or target ROAS. The duration can be very short, but it depends on a lot of factors.