SRFIs are modules, which are portable across different scheme implementations, also there is a process for proposing them. BTW, guile contains only subset of it.
ice-9 are basically guile modules, which could be (guile ...), but they are (ice-9 ...).
https://srfi.schemers.org/srfi-process.html
https://lists.gnu.org/archive/html/guile-devel/2010-07/msg00046.html
@abcdw The Guile standard lib should be called `guile`.