life cycle

Extensions have two types of lifecycles, depending on what the extension is for.

Application life cycle
Valid from the start to the end of Next Design (extension common to Next Design regardless of the project)
Project life cycle
Valid from creation or opening to closing of the project (extension for a specific project)

Extension execution flow

The general flow from loading the extension to executing the process is as follows.

Launching Next Design loads the manifests, scripts, and DLLs of all extensions located in the given folder.
Application lifecycle extensions are enabled and the ribbon tabs and buttons defined in that manifest are added.
When you open a project in Next Design that has a target profile for a project lifecycle extension, that extension is enabled and the ribbon tabs and buttons defined in that manifest are added.
According to the extension point defined in the manifest, when the corresponding UI operation or event occurs, the command handler or event handler implemented in the extension is called.

See Deploying Extensions for the folder where the extension is located.
If you replace the extension or change the script while the Next Design is running, those changes will not be reflected.
All extensions (manifests and DLLs) are loaded when you start Next Design and are not reloaded while Next Design is running.
The extension script is also loaded at the timing of the first processing call and is not reloaded while Next Design is running.