Centralization describes a system where one organization controls the whole network. This is like Uber or Twitter. Decentralization describes a system where multiple organizations work together and there is no central point of control. This would be couriers unions or mastodon. In mastodon a single server can host thousands of users but no server controls the network. Then there is peer-to-peer and this would be like Matrix chat or a system where customers pay couriers as individuals. Here is a diagram:
https://teblocha.com/wp-content/uploads/2021/01/sas.pngA system where restaurants take responsibility for deliveries is in principle decentralized as well, but I would argue that being a restaurant and managing deliveries are generally different tasks. Without other controls that system would likely devolve to one central firm contracting our delivery workers to restaurants, which is essentially exactly what we have now.
The issue with worker protection is not really about the size of the organization, but about who has control and decision making power. I suppose I should have said a cooperative of delivery drivers instead of a union, because in the USA the Taft-Hartley act severely compromised how unions behave and centralized their power in union leadership. So that was my mistake in phrasing.
But I believe a cooperatively run delivery service would better look out for the rights of delivery drivers specifically, compared to restaurants who are more focused on managing food service and would be likely to subcontract delivery service to a centralized firm.
Or look we have it both ways and restaurants subcontract to a cooperative of delivery drivers. My point is delivery drivers should organize and collectively manage their jobs. Actually I think every firm in our economy should be a cooperative.