The build is performed with cargo build --release -Zbuild-std for a library crate.
This checks whether everything builds successfully (excluding binary linking, which often requires complex toolchains).
For no-std targets, Cargo's build-std logic is used for figuring out whether to build std or not.
On older targets (older than November 2024) this does not always work reliably, so some failed std builds there are simply no-std targets.