Now before I try that I would like to clarify something and tell me if I'm wrong. Bitcoin network "doesn't" exist. There are just some people running full nodes with some ip addresses and a port that share with other full nodes. When you install bitcoin core you have some default nodes that you follow (I think they're written on a txt file).
No, they're hardcoded in the code, specifically in chainparams.cpp... they're
the "DNS Seeds":
vSeeds.emplace_back("seed.bitcoin.sipa.be"); // Pieter Wuille, only supports x1, x5, x9, and xd
vSeeds.emplace_back("dnsseed.bluematt.me"); // Matt Corallo, only supports x9
vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org"); // Luke Dashjr
vSeeds.emplace_back("seed.bitcoinstats.com"); // Christian Decker, supports x1 - xf
vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch"); // Jonas Schnelli, only supports x1, x5, x9, and xd
vSeeds.emplace_back("seed.btc.petertodd.org"); // Peter Todd, only supports x1, x5, x9, and xd
vSeeds.emplace_back("seed.bitcoin.sprovoost.nl"); // Sjors Provoost
vSeeds.emplace_back("dnsseed.emzy.de"); // Stephan Oeste
vSeeds.emplace_back("seed.bitcoin.wiz.biz"); // Jason Maurice
These are what a node will attempt to connect to when they have no known peers... and they'll redirect the node to peers to try and connect to.
If I remove that txt file and change the ports (plus some changes on the difficulty because it's too high) then everyone that will download my altcoin core will have a different blockchain to mine right? (satoshi's genesis block will remain, and I really don't care for that)
You can't just remove that file... you'd need to modify the source code to use your own DNS Seeds... if you just removed them from the source code, chances are that you'll face issues with other parts of the code that relies on them being populated with something meaningful.
If you don't have any "DNS seeds", then anyone that would attempt to use your altcoin, would not be able to connect to the altcoin network unless they already knew the IP address/port# for a node already on the altcoin network. So they wouldn't receive any block or transaction information... and you would effectively have no network...
Finally, I'd suggest you ask your questions regarding that guide... in the thread for that guide!