I am being mocked by friends for not being on the Bitcoin train yet, so I am trying to understand it all. I get how btc blockchains basically works, but when someone says "if bitcoins are copied by splitting the blockchain" or something like that (I can't even quote it correctly...), what does it mean? Are there 'fake' bitcouns floating around out there? And what to do if you end up with some fake btc?
There are a lot of scammers but counterfeit bitcoin cannot exist. Scammers can try to trick inexperienced users with watch-only wallets, or fake software etc, but they can't actually counterfeit coins.
Since the bitcoin ledger is public anyone can check to confirm if a bitcoin address has funds on it. In most cases you just need a single private key to spend the funds associated with a bitcoin address, so simply verifying that you have that private key and the address associated with that key has funds you can be sure those funds are really there.
Never accept a wallet file or a private key in exchange from someone trying to sell you bitcoin, instead simply provide them an address you already know you control and have them send funds to you over the bitcoin network.
The "splitting the blockchain" reference you mention is related to what is called forking. As bitcoin is open source anyone can create and run network similar to bitcoin with the same or possibly different rules than bitcoin. There are also instances where developers will take the bitcoin blockchain and copy it to start a new coin, but uses the existing balances of all bitcoin addresses (Unspent Outputs). This is a forked chain where users that had funds on bitcoin, now also have funds on some new chain but usually at a much lower value than the original. These forked chains, created out of nothing, usually suffer economically as opposed to the original bitcoin. Although it is a copy, it is not counterfeit it's just another blockchain network.