ARM is a UK company that sells more CPUs than Intel and AMD combined. They dominate the mobile market. Literally all phone chips use ARM technology. Their GPUs are not bad for tablets and phones.
They been trying to get their CPUs onto the desktop and server markets for years, and they are beginning to get some success stories on both.
ARM main business model is the sell of IP. They fully design their CPUs along with a host of surrounding technologies, like the interconnect. In order to prove their design work and are fit for purpose they have to send to TSMC and other fabs. Small customers buy their IP as is and then integrate it onto their designs. Their most expensive license is the Architectural licence. This means they allow the buyer to completely redesign their Chios, while maintaining compatibility with the Software. Large companies like Apple, Nvidia, Amazon, Samsung and others buy this licence and produce chips like the Exynos, or graviton.