The keyword is "Filtering".
It sounds trivial, but is absolutely isn't!
Filtering Blended Data sources based on a custom field is one of the most challenging and confusing tasks to be done in Looker Studio.
At least, until blended data sources are entities of their own and can have custom fields created at Data source level.
Until then, we need a workaround!
In today's ~30min video, we blend two Ad accounts (Microsoft Ads and Google Ads) and then we will try figure out how to create a filter that only keeps the sources we need.
And finally, we'll try to understand WHY the workaround works, and in the process will learn a lot about how data blending, join types, and filters work in the background of Looker Studio.