How do you protect your code from a freelancer?

You can't. But if you product is modularized you can expose some modules to the freelancer not the whole product.

Only hire people with strong reference and want to maintain their reputation. Otherwise, you'll spend too much time trying to police them.

Hire someone you know. If you don't know anyone, then just check for references, and also try to create a good work relationship with that person once you start working with him/her. Nowadays is easier to find a lot of information about a person's reputation just by looking online...

In the end, there is really nothing that will prevent someone from stealing code if that is what they really want to do, that is why is a good thing in my own opinion to find people trustworthy and maintain a good relationship with them so you don't have to worry about these things every time you need to hire someone to do a small change to your code.

There is a difference.

You can protect your idea from developers, but cannot protect your code, some parts will always be reused but you should not care about code, its the idea that matters.

You need to be a developer yourself (or have at least one trusted developer at hand) to be able to protect your idea from freelancers.

Do the design yourself and target a modular structure. Be clear and precise with requirements as this will help when integrating.

Distribute module development among freelancers and keep the integration part for yourself (or your trusted developer) so only one person will have access to all the code and will know what is being actually done.

The good thing about modular structures is that most of your freelancers will not even have the idea what they are actually building.

This approach is very good and helps in scaling your application too. Only drawback is slightly larger initial costs.