Powerful apps like Notion, Airtable, and Roam are all powerful/flexible and require learning and expansion of use cases over time to wrap your head around them. Given that, why do they all just use an upfront onboarding?
The goals that users have six days in are going to be different than the goals they have 6 months in, so User goals change over time, and it needs to be the case that User skill level increases over time in order to match their changing goals. Apps with continued user involvement respond to changing user goals over time, so the app should continuously teach people how to use its app better.
Continuous onboarding is an onboarding that never stops. Users are constantly being shown how to be better at using the app in an attempt to increase User Involvement.
It should be noted that some apps have a more expansive range of potential use cases than others. The important thing to continuous onboarding is to think about Goal Resonance- how will a user discover that the app helps them accomplish a goal and execute the sets of behaviors that enable them to accomplish said goal? The user may have a lack of imagination as to what user goals they can accomplish, so how can we expand their conceptualization of the product?