Who can tell me what actually happen (In plain words) when I send BTC to another

[Metros]

Since there is no main-server, how does my wallet know where to send the BTC at?

[prometheum]

It uses a distributed ledger which is defined below.

This is an excerpt from investopedia.

A distributed ledger is a database that is consensually shared and synchronized across network spread across multiple sites, institutions or geographies. It allows transactions to have public "witnesses," thereby making a cyberattack more difficult. The participant at each node of the network can access the recordings shared across that network and can own an identical copy of it. Further, any changes or additions made to the ledger are reflected and copied to all participants in a matter of seconds or minutes. Underlying the distributed ledger technology is the blockchain, which is the technology that underlies bitcoin.

[xenotrunksx]

Your wallet doesn't really "send" BTC anywhere.  Roughly speaking your wallet software submits a request to update an online database (blockchain) to anyone that is willing to listen on the network.  People listening on the network (nodes) relay the update (transaction) until it is approved (added to a block by miners).  The signature from your private key verifies that the update request you submit is valid and you have the authority to modify ownership of that part of the database (blockchain).

.....I think.

[IadixDev]

When you "send btc" you sign a transaction with the btc you own as input, and the output(s) contain the script with the public key (destination address) corresponding to the private key to spend it.

This transaction is then sent to the network, and added to the memory pool to be mined in the next block.

When the next block is mined, it will be propagated to other nodes, and the bitcoin can be spent using the private key corresponding to the public key in the output script.

The btc are not localized specially anywhere, but all node keep track of all the spendable bitcoin associated with the script to spend it which contain the public key, or the hash of the public key.

Anyone who own the private key associated with the public key in the script can then spend those btc.

[Smarty14392]

It's actually quite simple. Your wallet is a collection of private keys, each identified by an ID. When you send bitcoins to someone, those are just assigned to one of those private keys. The information is stored in the database known as blockchain.