The word "inbuilt" means something that is built or designed within a system or object from the beginning. It is a part of the object's nature or is inherent to its functioning. Full definition
The smartphone is equipped with 4 GB of RAM and 64 GB of inbuilt storage with a microSD card slot. (gadgets.ndtv.com)
On the storage front, this device comes in 32GB / 64 GB of inbuilt storage with an expansion option of up to 128 GB via microSD card slot. (goodereader.com)
The company had launched the smartphone with 32 GB inbuilt storage in October last year. (maktechblog.com)