To make an eBook reflowable in InDesign, you need to use the "eBook Interactivity" feature. This feature will allow you to add interactivity to your eBook, such as adding links, bookmarks, and audio/video files. To use the eBook Interactivity feature, follow these steps:
In InDesign, create a new document and choose the "eBook Interactivity" workspace.
Select the "Interactive Objects" panel and choose the "Link" tool.
Draw a box around the text that you want to make into a link.
In the "Link Properties" dialog box, select the "File" option and choose the file that you want to link to.
Click the "OK" button to close the dialog box.
Repeat steps 3-5 for each link that you want to add.
When you're finished, export your document as an EPUB file.