Here's a piece from The Repository focused on Yoast and Whodunit and why they contribute to WordPress core as well as the features they’re pursuing.
Some notable stats are mentioned in this post where Jb Audras breaks out the companies that contributed the most employees to WordPress 5.5‘s development:
- Automattic: 82 people
- Yoast and Whodunit: 10 people each
- Human Made, 10up and WP Engine: 8 people each
- Google and rtCamp: 6 people each
- WPMU DEV: 5 people
- Bluehost, GoDaddy, iThemes, and Audrey HC: 4 people each