ONDC being an open network means that buyers and sellers can transact on ONDC irrespective of whether they are attached to any specific e-commerce portal.
For example, even if a seller X is registered on platform A, while the consumer is registered on platform B, the consumer can directly purchase products of seller X without registering on platform A from the ONDC network.
The ONDC platform lies in the middle of the interfaces hosting the buyers and the sellers. Currently, the buyer side interface is being hosted by Paytm, whereas the seller side interface is being hosted by other players like GoFrugal, etc.
When a buyer searches for an item on the Paytm app, the app will connect to the ONDC platform from where ONDC has gone live, which will connect it to seller side interfaces that will list all the companies from where you can buy the particular item.
Similarly, with ONDC implemented, a user searching for a Bluetooth headset on Amazon would also see results from Flipkart on the Amazon app.