Need help in SP2016 migration.

Question

I have sharepoint 2013 web applications with multiple site collections...in these some of the site collection has their own dedicated content db assigned to them. Now I am migrating the sharepoint 2013 web application to sp 2016 using sp-mount command..... As we sp-mount command takes "webApplication" as parameter not site collection.... so how can I Migrate site collection to dedicated content db after root level site collection migration????