https://github.com/google/oss-fuzz/tree/a21768ce6a5056d27f82...
I thought they mostly fuzz the loaders, which are the most critical, apparently the found bugs present a picture of a pretty wide coverage:
https://bugs.chromium.org/p/oss-fuzz/issues/list?can=1&q=Lib...
As of June 2021, OSS-Fuzz has found over 30,000 bugs in 500 open source projects.