Which is the correct way to migrate content from single WP to WP multisite

The WP importer should do the job for you. You may need to install it first.

Backup. Backup. Backup. Then, from your current single site: Tools > Export > All Content > Download

Then from the new location on your multi-site, Tools > Import > locate the export file and adjust options to your needs.

Note – you will need to still take care of settings like widgets, setting up your menu locations and possibly a few other tasks depending on how your theme was written.

There are many horror stories out there about the WP import/export tool. Some are true but most problems occur (in my experience) when doing partial content. In this case you are moving everything but your mileage may vary.