Use these files in one of these basic ways:
They can be read, producing first the device descriptor (18 bytes) and then the descriptors for the current configuration. See the USB 2.0 spec for details about those binary data formats. You'll need to convert most multibyte values from little endian format to your native host byte order, although a few of the fields in the device descriptor (both of the BCD-encoded fields, and the vendor and product IDs) will be byteswapped for you. Note that configuration descriptors include descriptors for interfaces, altsettings, endpoints, and maybe additional class descriptors.
Perform USB operations using ioctl() requests to make endpoint I/O requests (synchronously or asynchronously) or manage the device. These requests need the CAP_SYS_RAWIO capability, as well as filesystem access permissions. Only one ioctl request can be made on one of these device files at a time. This means that if you are synchronously reading an endpoint from one thread, you won't be able to write to a different endpoint from another thread until the read completes. This works for half duplex protocols, but otherwise you'd use asynchronous i/o requests.