There are several ways to generate Smart Preview files:
Import
When you import new images into your catalog, select Build Smart Previews (Import dialog > File Handling section). Smart Previews are created for all the images imported into the catalog.
Export
When you export a set of photographs as a catalog, you can choose to build and include Smart Previews in the exported catalog. Click File > Export as Catalog, and then select the Build/Include Smart Previews checkbox.
On the fly
You can create Smart Preview files on demand. Select the files for which you want Smart Previews, and then click Library > Previews > Build Smart Previews.

Please check out the link below For detailed instructions on how to create Smart Previews:
https://helpx.adobe.com/in/lightroom-classic/help/lightroom-smart-previews.html