Globally, about 270 million personal computers (including laptops) [1] and 1.5 billion smartphones [2] are sold per year. Apple makes up about 17% of global smartphone market share [3] and 55% of US market share [4].
How can Apple "reign supreme" over the industry, to the point where they're facing significant anti-trust pressure, while also having negligible marketshare? Clearly there is more to this supremacy than raw marketshare.
US marketshare and mindshare among the (US) people who decide such things. If all the relevant politicians and judges and their friends and families (presumably all wealthy) have iPhones...
As well as having 55% US marketshare as other posters have mentioned, they account for 42% of global smartphone profits despite only having 17% marketshare.
In the consumer market, maybe. In reality you are only taking into account your country, in all other countries macs are rare, and are typically only used by creators (photographers, musicians, video makers, etc) or by someone that develops iOS applications (because you don't have an alternative, or legally you don't, a lot of people just have hackintosh VM to compile).
Nowadays a lot of people that used a mac went back to Windows, because let's face it, it's more stable, it's faster, and consumes less resources. I once was a macOS user, and the latest release was so slow on my 2015 macbook pro (that still is a pretty good hardware) that made me format everything and install Linux on it. Now it seems to me that I have a brand new computer...
In the professional market there is only Windows. In every big company everything is Microsoft, Windows, Office, Active Directory, Exchange, Windows server, Azure. Probably the reason is the IT department likes Windows more since it's easier to maintain than macs.
Regarding iPhone, they are not so diffuse as they are in the US. In my country you have a 19% iPhone, and it's one of the European countries where they are more prevalent, and still it's a minority. Most of the people uses Android phones, and doesn't want to spend more than 200 euros for a phone.
Probably in the US iPhone are so diffuse because you usually get them with your mobile plan, while in my country no one buys a phone that way, since you will end up paying it like double the price and you loose the freedom to change the provider as you wish.