I wanted to confirm the visibility of guest accounts when using SharePoint search vs Microsoft Search.
As per my testing the default search box that appears at top is using Microsoft search and the behaviour is "Guest accounts do not appear in search results".
Custom search-based solutions such as 'PnP Search Webparts', can be built using REST APIs that target SharePoint search e.g. https://<tenant URL>/_api/search/query. In that case, guests are included in search results.
Hence, in summary, guests are Excluded when doing people search using Microsoft Search whereas Included when using SharePoint search.,