Sebastian Falbesoner is funded by Brink for code review and testing: improving test coverage, extending the test framework, and working on the networking, peer-to-peer and wallet parts of the Bitcoin Core codebase.
That is the work the project itself names as its constraint. Bitcoin Core's README states plainly that testing and code review are the bottleneck for development, that the project receives more pull requests than it can review on short notice, and that this is security-critical software where a mistake can cost people a lot of money. Almost nothing in the wider software economy pays for review of someone else's patch, which is why grant funding for it exists at all.
He has described the value of the arrangement as including the part that is not money: the regular engineering calls and events Brink holds, and being able to learn from the other grantees.