Results from subsites will show up in top level site by default. When you are in subsite level and perform a search in Microsoft 365 search bar at the top, you will get results from within subsite but a breadcrumb is displayed at top to clearly indicate that and also helps you navigate to top level site and also to search results across organization.
Microsoft 365 search is scoped by default to run within the current context. If you are browsing a document library, and use search bar at the top, the initial scope will be within that document library.
