-Bitcoin is secure, look this post
Let's say we have a trillion planet Earths. On each Earth, there are a trillion people. Each person has a trillion computers. Each computer generates a trillion keys a second. All these computers have been creating a trillion keys per second since the birth of the universe 13.7 billion years ago. 10^12 * 10^12 * 10^12 * 10^12 * 60 * 60 * 24 * 365 * 13.7 * 10^9 = 4.3*10^65. This means thay they would have so far generated approximately 0.0000000004% of all private keys.
-Bitcoin is decentralized, any miners who mine Bitcoin and validate the transaction are proving anyone can take a part in Bitcoin network.
-Blockchain is just a public ledger, where anyone can see any transactions. If you're referring to Bitcoin block, where it's mined and stored on the blockchain, then it's also secure since you can't add random block to the blockchain. You need to find and solve the right block, 51% attack is almost impossible when most of Bitcoin already being mined and the difficulty are increase a lot. In case if you want to learn about this
https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch08.html