The SharePoint API only accepts access (bearer) tokens that have been acquired using client assertions. This article explains how to do that on protcol level.
ReadA term in SharePoint Taxonomies can have custom properties. PnP Core has now the ability to query terms by custom property value.
ReadIn SharePoint Online only a predefined set of properties gets synchronized from the Azure Active Directory User object to the SharePoint User Profile. This article shows how to add additional User Profile properties using a timer-triggered Azure Function and surface them in SharePoin Search.
ReadTroubleshooting search in SharePoint Online can be hard. This article elaborates all reverse engineering means I know of.
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