Trichromelibrary was suddenly downloading from Google Play Store

From Shared Libraries on Android which explains
native code in Chrome on Android

• Android L & M need ChromeModernPublic.aab


• Android N, O & P need MonochromePublic.aab


• Android Q needs TrichromeChrome.aab + TrichromeLibrary.apk

(Emphasis added)

Further down the page it explains what these packages do (in developer speak). So, it's perfectly safe and legit, as it is a part of Android Chrome - nothing to be worried about.

Edit: From Android Police

... Chrome no longer appeared an option in the WebView picker (found in Android's Developer Options). A Google engineer explained that Android 10 uses a new implementation called 'Trichrome':

"Chrome is no longer used as a WebView implementation in Q+. We've moved to a new model for sharing common code between Chrome and WebView (called "Trichrome") which gives the same benefits of reduced download and install size while having fewer weird special cases and bugs."