A software development kit broadly refers to a collection of related documents, examples and tools to help develop a specific type of software.
Software development toolkit is a collection of development tools used by software engineers to create application software for specific software packages, software frameworks, hardware platforms, operating systems, etc. Generally speaking, SDK is the SDK used to develop applications under the Windows platform. It can simply provide some API files for a programming language, but it may also include complex hardware that can communicate with embedded systems. General tools include utilities for debugging and other purposes. SDK usually includes sample code, supporting technical notes or other supporting documents to clarify doubts about basic reference materials.
In order to encourage developers to use their systems or languages, many SDK are provided free of charge. Software engineers usually get software development packages from the target system developers, or download them directly from the Internet, sometimes as marketing tools. For example, the marketing company will provide the component SDK for free to encourage people to use it, which will attract more people to buy its components because they can program for free.
The SDK may come with a license to prevent it from developing software under an incompatible license. For example, providing a proprietary SDK by a product vendor may conflict with free software development. GPL can make SDK almost incompatible with proprietary software development. SDK under LGPL does not have this problem.
Back to your question, 9 1sdk and 360sdk are special sdk packages provided by specific websites (in these two examples, 9 1 and 360 websites). Engineers who downloaded this package can carry out secondary development and develop mobile phone software that meets the requirements of this website.