They are complicated, but standardised and commoditised. Ubiquiti, for example, sells an ONT (fibre modem) in a SFP form factor for US$39 [1], or a little standalone unit with an Ethernet port for US$49 [2].
For comparison: you can bring your own DOCSIS modem to a cable network, even though all the houses on the street are connected to the same cable and you could jam it, or send a voltage spike to break everyone's modem.
Not very familiar with DOCSIS and cable; the story I'm getting from my nearest friendly LLM is that while you could bring your cable modem, it'd have to be a pre-approved model, and that the firmware and configuration would be under ISP control, unlike with DSL modems. Is that wrong?
By law the demarcation is a passive one; the provider is not allowed to mandate you operate ANY of their active hardware. If they want to sell you internet only via e.g. RJ45 Ethernet they better consider asking your landlord to rent them space and power and Cat.5(+) wiring access to put a switch/router, because by law they can't dump that on you the residential apartment renting customer.
You may either rent/buy a device from your ISP, or you may bring your own, at your discretion. ISPs are required to accept all devices, of course if your device kills the network segment, they will kill your connectivity. But they can't refuse to let you connect.