How can I prevent my partner from tracking my iPhone using a SIM swap?

The SIM has nothing to do with it. Your partner would need to either have your Apple Account Password, or you are sharing your location with them. If they know the Passcode to your device, they may have set up sharing your location without you knowing.

• Find My and location sharing - Apple Support

Another possibility is that they are "gaslighting" you by saying you are being watched and tease you with some information they know to try and convince you. Not uncommon.

supercondr wrote:

You can add a password to lock your Sim, that way if the sim is swapped, it requires the Sim passcode to allow.

This passcode is different from your passcode for your Apple ID.

If your using a eSIM on new phones, they don’t have a physical sim preventing this from occurring.

It's hugely unlikely there has been a SIM swap. They would easily know as they'd be getting the partners phone calls and they wouldn't be getting their phone calls.

You can add a password to lock your Sim, that way if the sim is swapped, it requires the Sim passcode to allow.

This passcode is different from your passcode for your Apple ID.

If your using a eSIM on new phones, they don’t have a physical sim preventing this from occurring.

Swapping out a physical SIM means the phone would make calls on your partner's cellular plan and receive calls to his/her phone number. That's all a SIM does. It does not, in any way, store personal information.

Surely if she swapped SIMs you would have each others phone numbers.

In the USA newer iPhones don't even have physical SIMs any longer.

Since what you asked isn't happening, it seems you and your partner have trust issues, which are outside the scope of this forum.