AFAIK, blockchain.info has hired 1 employee (although I can't recall which whop) and there was a Xapo listing for a Bitcoin Core developer some time ago (I'm not sure what happened with that). Other than those two, I haven't seen other examples of companies doing this.
Last I heard, Sjors Provoost works for Blockchain.info, Anthony (AJ) Towns works for Xapo, and Jim Posen works for Coinbase. Blockstream employees Pieter Wuille, Jorge Timon, Gregory Sanders, and several other contributors (plus two C-Lightning devs) Several companies also help support the Media Lab's Digital Currency Initiative (DCI) that employees Wladimir van der Laan and Cory Fields (as well as several other open source Bitcoin contributors who don't normally focus on Bitcoin Core).
The other major source of employment for Bitcoin Core work is ChainCode Labs.
(I could be forgetting some companies; if so, sorry.)
I thought it was Sjors, but I didn't want to spread potentially false information as I wasn't completely sure. Well yes, Blockstream is implied for everyone who's been around for a while (hence why I didn't mention it). I guess it wouldn't be bad to keep a list like this somewhere; maybe the community could create some pressure in order to get other big companies to at least hire 1 person to work on the reference implementation. This also helps with *development decentralization* (me recalls the bcash nonsense 'Blockstream = most commits' or w/e).
"Bitcore © BitPay, Inc. Bitcore is released under the MIT license." Stabbing my eyeball with a fork would be more pleasant than using an implementation made by a malicious actor.