I assume this is in the SRE book, but a tier one product like the identity service will have global SRE coverage (i.e. at least three SRE teams so that there is always an SRE group for whom it is daytime holding the pager). Devs are often involved in diagnosis, but are less often required for mitigation, as the mitigation is almost always to revert whatever change caused the problem. This is a simplification of course, but it gives an idea of the general pattern.