The Export Manager displays a list of the last few projects you've exported from Construct. This helps you find an exported project again if you forget to save the resulting file, or otherwise lose it. It can be opened via Menu►View►Export Manager. Note only the last few exports are kept and old ones are automatically deleted, so don't rely on it to keep everything - be sure to save exported files at the time you export them.
To download a previous export again, select it in the list and click the Download button in the toolbar. If you need to free up storage space you can also manually delete saved exports with the Delete button in the toolbar.
Paid plans only If you export for Android or iOS and select the Cordova project option, the result is a zipped Cordova project. You can then choose to build it by selecting it in the Export Manager and clicking the Build button on the toolbar. This will then take you through the steps to build a mobile app with Construct 3's build service. Note it's quicker to select one of the other build options at the time you export for Android or iOS, but this can be useful if you forget, or want to run more than one kind of build.