ARM generally makes 32 and 64 bit RISC architecture CPUs. They are known for their significantly less power consumption making them suitable for portable devices. The reason for less power consumption is the RISC (which stands for Reduced Instruction Set Computer) architecture. It only has very few necessary instruction set which allows the cpu to be smaller.
Compared to this Intel’s x86 cpus are made of CISC(Complex Instruction Set Computer) architecture. This means that those cpus has a longer list of instructions they can execute but that also means a bigger cpu and more power consumption.
Recently there has been a trend of replacing even servers with ARM cpus for it’s power efficiency as data centers generate a lot of heat and the cost of cooling is significant. Another reason is the x86 instruction set is licensed by Intel and they only allow few companies like AMD to use those instruction set under an agreement and recently even modified the agreement to prevent transfer of technology in case AMD is acquired by google. All these things make other companies uncomfortable so they all want a design they can easily get access to and modify according to their needs.