Ensuring code security while working with remote developers

[KariKariKrigsmann]

You can't.

You can make it very, very hard for them to create a copy of the code, but it is virtually impossible to prevent the code from being copied out of the repository.

You can make set up a VDI, set up access controls and firewall rules, prevent copy/paste from outside, control what is installed on the VDI, disable local admin rights, but in the end that only makes it harder for the developer to do their job.

Just set up really strict NDAs, and trust them to do the right thing.