In general, master copies of SSRS Content types items like Reports (.rdl), data sources (.rsds), semantic models (.smdl) etc. are stored in the SharePoint content DB and cached on-demand by SSRS in the RS catalog. Additionally, report / data source metadata like subscriptions, schedules, snapshots, stored credentials etc. are exclusively stored in the RS catalog. Having said that, all these reporting items stored or cached within the RS catalog are associated / linked with where they are stored in SharePoint. These associations generally comprise the SharePoint site GUID, relative paths wrt to the SharePoint root site, etc. Hence, in general a large portion of data stored in the Report Server catalog DB in SharePoint mode is relative to or dependent on SharePoint content/ sites. As a result, simply migrating the backed up reporting services database into another SharePoint farm is not going to work in every case.
SSRS migration (with backup and restore) needs to be step-by-step with SharePoint configuration / content DB migration. Hence the best practice / supported means: