You can collapse a symbol for the Subprocess type. This way the process modeled in the subprocess is assigned to the subprocess and the subprocess is collapsed.
Click Collapse subprocess. The process contained in the subprocess is assigned to the object and, at the same time, is displayed as embedded in the object, which is indicated by a plus sign. If a process is not yet assigned to the subprocess, the Create assignment dialog opens in which you can assign a new BPMN process.
Click the subprocess again.
The process contained is assigned to the subprocess and can be opened by double-clicking the assignment icon.