What is the view and engagement coverage for each platform?

YouTube
The API does not provide a fully unified definition.

For standard videos: “The number of legitimate views for your channels or videos.”

For Shorts: “The number of times the video has been viewed.”

As of March 31, 2025, for Shorts, the viewCount field will indicate the number of times the video was started or restarted, with no minimum watch time requirement.
Reference: YouTube API

Facebook
Generic definition: “The public view count of the video.”
Reference: Facebook Graph API

Instagram
Same case as Facebook, but restricted to Reels via the Business Discovery API.
Definition: “View count for Instagram Reels.”
Reference: Instagram Media API

TikTok
There is no detailed official documentation, but according to TikTok’s own website:
“Number of times your video started to play. For each video impression, plays are counted separately and replays are excluded.”

In other words, each playback start is counted, with no minimum watch time requirement.
Reference: TikTok Ads Help

Twitch
Direct definition: “The number of times that users have watched the video.”
Reference: Twitch API

Engagement:

YouTube: Likes and comments
Instagram: Likes and comments
TikTok: Likes, comments, and shares
Facebook: Likes and comments
Twitch: Currently, the Twitch API does not provide engagement data