3G, 4G, 5G and 6G simple differences
Seems like a simple question but I've searched and not found good answers.

This is really not much of a simple question. You're basically asking about &quot;what happened in ca 23 years of rapid technological advancement&quot;; your four things are right, and they are important, but it's much more than that!

It comes down to signal to noise ratio equation

That is, if you had perfect channel knowledge (to make your channel perfectly flat), and perfect channel coding (to make use of that SNR).
Neither is the case!

Differences … Frequencies ranges used

Not really any. OK; 5G New Radio opened up the millimeter waves. Other than that, not much happened: Differing allocations in different countries ranging from ca 800 MHz to ca 3.4 GHz. The availability of these really has less to do with the standards than with local laws. It's a regulation thing, not a standardization thing!
Communication bandwidths of course changed, but at the same time, more modern standards tend to be able to divide the spectrum in a more finely-grained manner, so this is hard to put into a few words. Basically: Read the 4G physical layer standards, that shows how diverse things are.

Differences … Modulation types

oh. Like, basically, everything. 3G used to be a CDMA system in most cases (you'll notice that I use &quot;most&quot; here: it's by far not as uniform as people think). That comes with its own challenges to equalization, channel sensing, and power control.
This leads to different choices in modulations contained in the chips than what you'd choose in an OFDM/TDD or OFDMA system like 4G and later.

Differences … Signal strength (amount of towers)

That makes no sense. The standards don't define how many towers you build. It's the regulations that define how much power a tower can transmit, or a handheld. And it's mainly economic reasons why you put up more or fewer towers. The keyword you'd want to research here is network densification.

Differences … Spectrum re-use (smart antenna/comm systems)

Well, again, you have to consider the differences in multi-user access: CDMA vs OFDM/TDD or OFMDA.
Then, 4G saw the introduction of MIMO techniques (which are much easier to build with OFDM).
That goes well with smaller segmentation. (&quot;smart comms systems&quot;: um, I guess everything is kind of smart?)

can't think of anything else that makes a fundamental difference to the data rate, so it must be mainly these parameters that change between 3G,4G, 5G and 6G...

Nope! These are really important, but would play a minor role without changes elsewhere in the system. Because I can't write a book series on 3G/4G/5G, just a few things to give you ideas about things that gotten better, which have a significant influence on throughput:

Channel Codes (!!)
Equalizers / OFDM equalization
Time-Variant channel estimation / prediction
Diversity techniques / MIMO
Amplifier linearity
Self-interference cancellation
Reduced signalling overhead
Interleavers
Checksumming
Retransmission Protocols / ARQ
Local Buffering at Base Stations and larger user equipment memories
Latency of control &amp; ACK / Allowing for contention windows to work with that
Resource Reallocation
Network Planning tools
Immensely reduced power demand of silicon hardware leading to availability of way more advanced versions of all the above on both user equipment and base station site
Immensely reduced upfront cost for basestation deployment, meaning that cells with fewer users make sense economically now, which wouldn't have been feasible before

This list is far from complete, and you really mustn't assume some of these points play a minor role in your youtube streaming speed just because they seem to be higher up the stack than others or even of a financial, non-technical nature.