Ethereum is a network of nodes, and in a fully decentralised world you’d be running your own node locally and connect to that when making transactions or querying for data.
However, not everyone can or wants to run their own node, and because of this we have infrastructure providers like Infura that allow you to connect to the Ethereum blockchain network using their nodes.
Using Infura rather than your own node is super simple. Because you keep your own wallet private keys, switching between these services is as easy as pointing Web3j to another node URL.
You can sign up for Infura at no cost, using their free core account offering. After doing that, you’ll be able to create a project to obtain a project ID. The Infura URL will follow this structure: https://<network>.infura.io/v3/YOUR-PROJECT-ID
And that’s practically all there is to it, very straightforward.
Using a Project Secret
The previous URL didn’t need anything more than the project ID to enable the connection, but Infura also supports having a project secret. Such a project secret is passed to Infura using HTTP Basic Authentication.