It's nebulous. Apps that employ(/contract/whatever) their delivery drivers separately from the restaurant have a clear picture, but when you leave a tip on a site like Seamless the money goes to the restaurant. You certainly hope the restaurant is distributing that to the staff, but you don't know for sure.
Interesting, in my experience (in NYC) Seamless and the like absolutely do not employee the delivery drivers. They are employed by the restaurants. Though apparently in some areas they do employ drivers: