Once you understand how your system fits in to the overall IT environment, a really useful next step is to zoom-in to the system boundary with a Container diagram. A "container" is something like a server-side web application, single-page application, desktop application, mobile app, database schema, file system, etc. Essentially, a container is a separately runnable/deployable unit (e.g. a separate process space) that executes code or stores data.
The Container diagram shows the high-level shape of the software architecture and how responsibilities are distributed across it. It also shows the major technology choices and how the containers communicate with one another. It's a simple, high-level technology focussed diagram that is useful for software developers and support/operations staff alike.
Scope: A single software system.
Primary elements: Containers within the software system in scope.
Supporting elements: People and software systems directly connected to the containers.
Intended audience: Technical people inside and outside of the software development team; including software architects, developers and operations/support staff.
Notes: This diagram says nothing about deployment scenarios, clustering, replication, failover, etc.
Both the Single-Page Application and Mobile App use a JSON/HTTPS API, which is provided by another Java/Spring MVC application running on the server. The API Application gets user information from the Database (a relational database schema). The API Application also communicates with the existing Mainframe Banking System, using a proprietary XML/HTTPS interface, to get information about bank accounts or make transactions. The API Application also uses the existing E-mail System if it needs to send e-mails to customers.
The dashed line represents the boundary of the Internet Banking System, showing the containers (light blue) inside it.