Serious question, how do Spurs have this much of a global following? Absolutely nothing club IMO.
Before televised games, Tottenham and Chelsea were the two main clubs in London in terms of local following. That's why the London derby name was carried by those two.
Liverpool and Manchester United also had strong support in London, which became cyclical (70s, 80s Liverpool and 90s, 00s Man Utd)
Chelsea had the upper class from almost the entire city and the majority of lower class in the western side of London. That's Hammersmith, White City, Ealing etc..
QPR also share some of those regions (ie White City/Shepherd's Bush) but they were more of a borough club.
Both Tottenham and West Ham have a large support from lower class Londoners as well as immigration to North and East London respectively. Whereas Millwall also has a lower class fanbase, albeit a different type of worker status than West Ham.
Arsenal was always a more random one. Lots of migrants became fans because it was easy to get to games as their core areas were Finsbury Park and Islington areas, which were more affluent and these people weren't really going to sports as much as the rest when things didn't go well. 90s successful runs made the club less localized and more London migrants started supporting them. It was viewed as an elitist club, even if they weren't attracting the Kensington and Knightsbridge folk that were preferring Chelsea due to proximity (although I'd bet there were more proclaimed Liverpool fans there in the 70s and 80s).
Arsenal had more fans in England than the other teams, but not so much in London. Would depend a lot on each study made, you couldn't say with accuracy what the actual ranking looks like.
A lot depends on the decade and the timing of such question.
This is also why you see clubs like Chelsea, Tottenham, West Ham, Millwall, Crystal Palace and even Brentford, have some sort of identity attached to them, whereas Arsenal just doesn't.
Away fans to Arsenal were looking for trouble at Tottenham, West Ham, Millwall or Chelsea, in case they were in town, not Arsenal.
Today there are probably more Arsenal fans based in London than Tottenham and Chelsea, but if you cancel out anyone who doesn't have ties 2 generations (or even 1) to the city, it'll be surprising to see Arsenal in the top 3. Even behind West Ham.
But if we count non-London clubs, the #1 supported club in London is Manchester United.
Why does Tottenham have a global following?
They had Argentinean players in the 80s, so they were always looked upon preferably over there. There's also Son which is a huge factor for Korean support and the surrounding area.
As was said, it's an easy club to root for if you don't want to be called a gloryhunter. Truth be told, they've had some great players in the last 15 years, so there's this plus the video game factor as kids always like to play with their favorite players.
And the NFL business is helping them a lot nowadays with popularity.
But most importantly, you need to understand that this didn't arrive in a vacuum. Tottenham was the most supported London club for a long while, and given how it doesn't translate to success and they're still backed shows something. And it can become contagious, because the club indeed has some character once you get to know their story. Unlike Arsenal who were "made" in the 1930s and went on an almost 50 year hiatus until they became relevant again.
I know a lot of people who became Tottenham fans in the 90s because of 1-2 players or for whatever random reason. That's almost the equivalent of becoming a fan of Monza in the 2020s...