Featured Images are used by:
LinkedIn, Twitter, & internal use within your WordPress website.

First Image within Post Content is used by:
Instagram, Facebook, Mastodon, and Pixelfed.

Notes:
Mastodon and Pixelfed image syndication behaviors can be customized from within the ‘Share on Mastodon’ and ‘Share on Pixelfed’ plugins, respectively.

Instagram, Facebook, LinkedIn, & Twitter image syndication behaviors are fixed and non-configurable.