How to broadcast a transaction I think this question is not only matter of bitcoin but also of every other crypto currency.

My question is if I lets say have an app or wallet or website. And I want to send money to certain address, lets say I will scan QR. Great I know the address but how to actually push the transaction to the blockchain? How I will release such info? Lets say that I would be able to see at https://www.blockchain.com ? Thanks for every answer. I am looking forward your responses.
You need to be running a full bitcoin node like Bitcoin Core, or use a wallet that connects to a bitcoin node. Bitcoin nodes connect to multiple other nodes and distribute the transaction and block data between each other according to the Bitcoin protocol.

Blockchain.com is also running a bitcoin node, and will receive the transaction through the network and display it on their site.