Typically in grant work you submit a complete proposal with milestones and roles defined, and receive payout over time to cover the costs in the plan, or some part of them. It's earmarked money.

In more established non-profit areas there's usually also quite some compliance overhead and audits to be passed, so this can be someone's fulltime job on the org side. FOSS backing orgs are typically smaller and less experienced, so donors have so far found ways to make things easier for them and give more leeway.