Why do so many different broadband providers use the same network?
Because in most countries the physical access network is a natural monopoly — nobody is going to dig up every street several times over — so regulation separates the company that owns the wires from the companies that sell service over them.
The structural problem. Building a line to every premises costs an enormous amount and the cost barely falls with a second entrant, so a purely competitive market produces either one provider or wasteful duplication. Regulators addressed this by opening the incumbent's network to competitors on regulated terms.
The mechanisms, in increasing depth:
Resale, where a competitor sells the incumbent's product with its own billing and brand, and controls almost nothing.
Bitstream access, where the incumbent carries traffic from the customer to a handover point and the competitor provides everything beyond it. The competitor controls the internet service but not the line.
Local loop unbundling, where the competitor installs its own equipment in the exchange and takes over the copper pair itself. Maximum control, maximum capital cost — and it was the basis of most price competition on copper.
Duct and pole access, which matters most now: competitors lay their own fibre through the incumbent's existing ducts and up its poles, avoiding the civil engineering that dominates the cost.
What follows for you as a customer:
The engineer who visits may work for the network, not your provider, which is why fault resolution routes through a wholesale process and why providers sometimes genuinely cannot expedite it.
Line quality is identical between providers on the same network. Differences come from contention, backhaul capacity, peering and traffic management — which is where providers genuinely differ.
Switching is easy within one network and slower between networks.
Genuinely separate networks exist — cable operators and full-fibre altnets built their own — and those are the only cases where changing provider changes the physical path.