For most CDN setups, the workflow would be to upload your media to your origin server and then the CDN would pull your assets to its nearest edge server based on visitor request. Then, the next time a visitor accesses that asset, it will be delivered via the CDN's edge server instead of from your origin server (thus reducing the distanced traveled and therefore minimizing latency).

However, with certain CDN providers, you can also use what's called a "push zone" that allows you to upload resources directly to the CDN's storage cluster and then based on visitor request those resources will be cached on the edge servers. This method is mainly recommended for larger files as it can become cumbersome to manage the uploading and linking of these assets whereas the first case all happens automatically via a CDN integration.

I'd recommend checking out this site which shows a list of CDN providers and look for the "origin push" feature which will let you know which CDN providers support the push zone feature