My previous job was working for $major_telco in the US, I was in network (not RF engineering). I left right as the "5G" train was starting, however I did get training and have pretty decent familiarity with the implementation plan and 3GPP release 15, the first release with the official New Radio (NR) spec. I also have a large understanding of LTE (3GPP release 10-14), so I'm happy to dive as deep as anyone would like.

For the details below I'm going to not use the term "5G", 5G like 4G is marketing. The technical specifications that more or less make up "5G" are the 3GPP standards releases[1]. The 3GPP is the standards body that ratifies the wireless network standards that nearly the entire world uses. For this discussion I'll ignore alternatives since "5G" effectively means the 3GPP standard.

The standard of 3GPP Release 15 (and newer) are improvements and build off the existing standards of LTE (releases 8-14). Its an evolution of the standard, much like 3GPP Release 8 (first LTE release) was an evolution on Release 5-7 (HSDPA-HSDPA+). While release 15+ are evolutionary, they are not revolutionary in that there is no magically discovered new physics behind it. The improvements largely lie with increased support for higher modulation levels (256 QAM was introduced with Release 14 LTE-Advanced), increased spectrum efficiency (variable sized framing allowed across difference devices and upload/download), mixing upload/download division types (i.e. using TDD[2] for download and FDD[2] for upload), improved MIMO (up to 64x64 in massive MIMO), improved beam-forming and additional frequencies.

Some of these improvements in Release 15+ were available in Release 14 or unofficially rolled out in release 14 + NR draft. I know one carrier that was pushing 64x64 MIMO for TDD LTE.

The new frequencies, many in the "millimeter wave" range, will help with with congestion in the "football stadium". There are two main limitations in high capacity events, the first is backhaul. Have to connect the stadium back to the core, and this is _always_ a bottleneck. The second limitation is available spectrum. No matter how many antennas you have in the DAS, there is a physical limitation to the amount of data that can be sent over the frequencies. The new millimeter wave help here, because while its very short range, its large width allows for a significantly higher number of concurrent connections.

The new frequencies, along with increased efficiency in existing frequencies, plus core changes are the main driver for the "latency" and "bandwidth" improvements. The "connected cars" and "connected IoT to cell network" are just marketing/sales departments pushing for new customers. The main "advantage" "5G" brings here is an increased capacity in the network to handle this.

A few other notes, unlike "3G"->LTE, the upgrade to Release 15+ for carriers will be a lot smoother. First, everyone is now on LTE, aka the precursor so there is no CDMA/EVDO networks that are incompatible that need rip and replace + compatibility modes (ehrpd). Second "NR" is designed to be compatible and multiplexable with existing LTE/LTE-Advanced enodebs, this means in one area you can have NR and LTE towers, and the NR towers can broadcast LTE for devices that are LTE only. This was not the case with the original eNodeBs, which could not handle backwards compatibility without physically separate BTS/nodeBs. Third, the new core for release 15 is designed with backwards compatibility with existing enodeb's. Unlike the previous transition which required a new core that was largely incompatible due to major design changes. So with "NR" RAN elements and existing LTE enodeb's the core can be seamlessly upgraded without having to run two complete networks for multiple years like in the LTE transition.

[1] https://en.wikipedia.org/wiki/3GPP#Standards

[2] TDD-> Time Division Duplex, FDD -> Frequency division duplex. Most LTE networks are FDD, a few (i.e. Sprint, Softbank, China mobile..) have certain spectrum they use as TDD. The difference is with TDD, you use the same exact frequencies for upload and download but you divide the by time. So basically t0->t2 is for download, t3->t4 is upload, etc. With FDD the frequency or "band" is divided into to two parts, one for upload and one for download. There is no time division for FDD but you lose of the size of the channel.