The C toolchains are registered with the standard type @bazel_tools//tools/cpp:toolchain_type. For details about C toolchains registered by default, refer to prebuilts/clang/host/linux-x86/kleaf. For details about Bazel's C toolchain registration, see this tutorial.
For details about C toolchain resolution in Kleaf, see prebuilts/clang/host/linux-x86/kleaf/README.md.
When --config=hermetic_cc is set (this is the default), for cc_binary, cc_library etc., the build system builds them against the execution platform (usually Linux x86_64) with the toolchain version defined by @kernel_toolchain_info.
To build binaries for an Android device, build the targets against the target platform by using --config=android* or wrapping the target with an android_filegroup rule. See build/kernel/kleaf/tests/cc_testing/BUILD.bazel for examples.
If --config=musl_platform is set, and the binary is for the execution platform (“host binaries”), the binary links against musl libc. See musl.md for details.
Kleaf uses Bazel's toolchain resolution to determine the C toolchain for kernel_build, etc.
Kleaf always uses the toolchain files (clang, ld, ar, etc.) from the resolved toolchain. These files are usually consistent across all platforms and architectures.
When --incompatible_kernel_use_resolved_toolchains is set, Kleaf uses flags from the resolved toolchain to determine USERCFLAGS, USERLDFLAGS, HOSTCFLAGS, HOSTLDFLAGS etc. for building the kernel. This requires kernel_build.arch to be set properly, because the flags are different for different architecture.
For implementation details, see kernel_toolchains.
A hermetic toolchain is registered with the type hermetic_toolchain.type (internally //build/kernel:hermetic_tools_toolchain_type).
All rules provided by Kleaf, including hermetic_genrule and hermetic_exec, uses this registered toolchain.
Any custom rules that needs the hermetic toolchain should find it from toolchain resolution. For details, see Hermeticity: custom rules.