I wouldn't say most computers are sold with their products anymore. 20 or even 10 years ago maybe, but AMD has a pretty big chunk of market share now, and Ryzen chips are AFAIK generally considered much better than Intel alternatives anyway.

AMD is their main competitor. ARM stuff also exists.

amd. there are fewer mobile options but unlike 6+ years ago they're actually a usable alternative in laptops.

For laptops, you can get AMD or Apple processors. Apple is more powerful and efficient and AMD is more compatible. Loongson may be a decent alternative for desktops in a couple of years if they aren't tariffed to hell. Qualcomm has released some ARM processors for laptops but the platform isn't mature yet, especially if you don't want to use Windows.

For desktops, there's no reason not to do full AMD. Dell makes some ryzen laptops but they don't have a GPU (so no solidworks). You won't really find AMD gaming laptops, but gaming laptops kinda stuck anyways (as someone who only owns them). You can fairly easily find AMD workstations but good luck if you're trying to find an AMD cadtop.

They've been losing marketshare to ARM for years. For cloud infrastructure servers (where the big bucks are) I know Amazon has been making a huge push towards ARM-based processors since the late 2010s — they offer them cheaper than servers with equivalent Intel chips, and in some cases they have better performance too, which is a major incentive for companies to switch. (Every company I've worked at has had a project to switch to the ARM-based servers.) Google just announced their own ARM-based processors for their cloud platform this year, and Microsoft has been offering them for a couple years too.

Part of it is that it's easier for these companies to customize (and manufacture?) their own ARM-based chipsets, which means they can offer them for a lower price vs Intel