Traditional building coverage systems throughout the 2000s and 2010s were built very similar to constructing an actual cell tower. One large, high-power transmitter was installed in a comms room and large thick coaxial cables were run through the building, branching off in to tens or hundreds of simple antennas that output signal. This architecture type is called "Passive IBC".
As the complexity of technology increased, with higher frequencies and multiple antenna techniques like MIMO, this approach became ineffective, and IBC began moving towards a "Hybrid" architecture. This involved the placement of intermediary medium-power transmitters placed on each level of the building and connected back to a central unit in the comms room via fibre optic or Cat6 cabling. Hybrid architectures still use large chains of coaxial cable to distribute antennas throughout each level.
The most modern solutions use an all-digital "Low-Power Active" IBC architecture. These deployments use low-power transmitters called "Coverage Nodes" placed throughout each floor or in key areas. Having integrated antennas, they can support complex antenna technologies like 2x2 and 4x4 MIMO, and much higher frequencies up to 6 GHz. They connect back to an Access Unit placed in the building's comms room, via fibre optic or Cat6 cabling. The use of fibre/ethernet compared to coax results in a significant cost saving (both in materials and labour) while supporting much more advanced technology.
Coverage Nodes can still use chains of antennas to improve the cost-effectiveness of deployment, or to reach into harsh environments like industrial cold-rooms, mining environments, and areas that are challenging to service.