Multiple wallets means I have to have [multiple] accounts;
The original creator of Bitcoin, Satoshi Nakamoto, wanted to create a currency for internet use which did not depend on any trusted third parties. Originally all wallets were software programs that you downloaded and ran on a personal computer. You would only have one wallet, not multiple.
There were no accounts, no exchanges or other businesses that you had to trust with your money. That was deliberate. You can still use Bitcoin like this today.
Over time people and businesses have seen an opportunity to offer services to people who either don't understand the original intent or who want to treat the currency not as a currency but as a speculative asset to be traded. These people have set up custodial accounts and have often described those accounts as wallets. These "custodial" wallets are not the type of wallets that the founder of Bitcoin intended.
You don't have to have multiple accounts. You don't have to have any accounts. You can use Bitcoin with no accounts whatsoever. Just download the wallet software of your choice, transfer all your Bitcoin to that wallet and close all your other accounts.
private key is used to decrypt the transaction.
The Bitcoin network does not use any encryption. So no decryption is necessary.
The private keys are used to provide a mathematical proof that you have the right to spend amounts of Bitcoin associated with a Bitcoin address (or more correctly, associated with a Bitcoin script that can usually be represented as a Bitcoin address).
Private keys are effectively used to prove ownership of money.
I also got to know that the key can also be changed.
Not really, however you can use a transaction to move money to the control of a different private key.
what is the actual key for the bitcoin account
The Bitcoin network doesn't know about or keep track of accounts. Many Bitcoin wallets don't have any sort of account concept.
If you have an account with some sort of trusted third party, the way that works is proprietary and may be different for each business.