Lots of companies do contribute to Debian and this has been happening for a long time. To start with Canonical employs people to contribute back. For a while HPE was donating large amounts of server hardware. Several companies (early HP, ARM etc) contributed lots of work on new ports like arm64, hppa, etc. Almost the entirety of the LTS effort is funded by companies. The current status is summarised on on the wiki:

https://wiki.debian.org/LTS/Funding https://wiki.debian.org/ExternalEntities