Sometimes, I would see some news says:

1. xxx service is build upon bitcoin or base on bitcoin blockchain
2. xxx servie is making use of current bitcoin network

3. And today I see a DNS project says that it use bitcoin blockchain:

   Its doc says:

   For example, a Bitcoin node may look at a Blockstack transaction and only see that bitcoins are moving from one address to another and that an unintelligible sequence of data has been attached in a data field (e.g. a field identified by OP_RETURN). Meanwhile, a Blockstack node will look at that data and will know how to interpret it in a way that updates the name database.

How do them mean build upon/ making use of ???

Here, assume X network with 5 node build upon bitcoin blockchain, I think it need:

1. has some bitcoin in each node(for making transaction)
2. if X network want to achieve immutable, it need send some bitcoin to another with some data(bitcoin allow to write additional data in a transaction).

Is this how them use bitcoin blockchain? But this way cost transaction fee... Or there is a free way to do such things?