SharePoint migration

Hello, I am interested in migrating SP on prem to cloud, but I have a couple of questions in mind: 

If I have a certain SP site on on-prem, and SP site with the same name on cloud (that was independently created not with a previous migration) what will happened with the content, permissions etc. if I decide to migrate the on-prem site to the cloud? 

For example, if only certain people could see the site on on-prem, and everyone else could see the site on cloud what will happened with the access after migrating?

If I have a SP list with 10 items in the cloud site, and a list with same name on on-prem with 5 items, will the other ones be deleted in the cloud after migrating?

I am interested how are those things handled?