How to split one Ethernet connection into three? Help!

Greetings, everyone. I am currently in the middle of renovating my home and have realized that I desperately need a complete network overhaul. My local hardware supplier suggested utilizing Cat6 cabling for those longer runs... Since the house is split into three separate living units, my goal is to get them all interconnected. Now, I have a decent grasp of high-voltage electrical work, but when it comes to low-voltage networking, I am somewhat out of my depth. I actually live in Germany, and honestly, hiring a professional contractor to handle an installation like this feels prohibitively expensive, so I would much prefer to tackle the project myself if possible.
So, here is the plan: I intend to run Cat6 through each of the three units. I was thinking about routing all the direct lines down to the ground floor and then perhaps consolidating everything into a single Cat7 hub if that is actually necessary?
Could someone kindly walk me through the most efficient way to structure this setup, keeping in mind that my longest cable run will be roughly 65 to 80 feet? 🤔 🤔
Much appreciated,

If you're looking for the absolute easiest way to expand your network capacity and get more devices talking to each other, just grab a switch. And for heaven's sake, make sure it's a Gigabit one.

Thanks for the help, though I should have mentioned from the jump that my service provider is Verizon, and I'm actually running my phone lines and fax through that same single cable. So, basically, the line hits the wall jack, and then splits out from there into both the router and the fax machine—does that change any of the variables here? 🙂
It might sound like a bit of a silly question to those of you who live and breathe this stuff, but I figured it’s much better to ask and poke around a little than to end up getting hit with a massive, unnecessary bill.

You just plug all those cables into a switch, run one line from the switch to the router, and you’re done. Honestly, three apartments isn't exactly a massive amount of computers. For basic everyday use, standard 100Mbit switches and Cat6 will get the job done just fine.

P.S.

At my place, I’ve got 50m of cable run out, ending at a switch that feeds three different PCs. Everything works perfectly. I’m on fiber, so there’s zero noticeable lag compared to just plugging a laptop straight into the router.

It’s all standard Cat6 cabling and an old AT&T router.

Perhaps I didn't frame my question quite clearly enough, though I realize that from a switch, you can certainly route connections to various computers and routers. What I am specifically wondering about is this: since all three apartments are currently under those old legacy contracts with Comcast (limiting us to 10Mbps), would it be possible to take a single incoming feed line, split it three ways, and then ensure each apartment has its own dedicated internet access? I want to make sure they aren't sharing the bandwidth, because given how those old contracts work, it actually turns out to be more cost-effective for us to maintain three separate 10Mbps lines rather than upgrading to a single 30Mbps plan—even if that sounds completely nonsensical from a logic standpoint, it’s just the reality of the billing here.
So, if we have just one main line coming in and then use a switch to distribute it to the three units while keeping the individual service contracts intact, would that actually function the way I'm imagining?

Highly unlikely...
Every single contract requires its own dedicated router that plugs straight into a wall outlet.

Much appreciated, thank you very much

it really depends on how reliable that 10mbps connection actually is and what people are trying to do with it.

if we're talking 10/10mbps, you could probably stretch one contract across three apartments if the users aren't asking for much.

but realistically, what’s the setup been like so far? was every unit running its own individual router?

I’ve been doing a bit of tinkering lately and I actually have a pretty solid offer on the table; one of my old contracts just wrapped up, so I’m planning to close that out and pick up a plan with at least 100Mbps speeds for just a few extra bucks—it seems like the perfect way to get everything sorted. My current setup involves switching between two routers and three different computers, so now I’m wondering if anyone might be able to recommend a reliable switch to tie it all together. It doesn't need to be anything overly high-end, really; we're just looking at four phones and three computers used mostly for scrolling through Facebook, watching YouTube, or streaming a movie here and there, so nothing too heavy or demanding on the bandwidth.

Honestly, your easiest bet is just grabbing whatever router the ISP provides. At least then you know it’ll actually do what they promised. If you go out and buy your own gear, they’ll just blame any connection issues on an "incompatible device" since it didn't come from them. That's just how the service providers work here. Some of them won't even let you use anything other than their own hardware.

And what do you mean by a switch, two routers, and three computers? I don't really get how you're planning to wire this together. Are you trying to run everything off a single 100mbps WAN connection? Or what? And why two routers? It's not like Lego bricks where you can just snap them together and hope for the best...