How can I restore my iPhone's battery health?

You cannot. Battery health only goes in one direction: down. When it reaches 80%, you need a new battery.

See here for some additional information:

When to charge your iPhone or iPad - Apple Community

Bikergofast wrote:

Thankyou for the reply, I had to charge and was in a pinch so I used my old Pixel charger and cable. The phone didn't stop at 80% so I thought I damaged it but it was the first charge cycle.

Charging your phone once with an OEM cable from a different major manufacturer is unlikely to cause a problem. It's the cheap cables you get from dollar stores or the convenience store connected to the gas station that are likely to be an issue.

Even if you have the phone set to stop charging at 80% (which I think is one of the silliest things Apple has ever added, but that's just my opinion), it will still occasionally charge up fully.

With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.

About Optimized Battery Charging on your iPhone - Apple Support

And for more information, see here:

When to charge your iPhone or iPad - Apple Community

Thankyou for that great link. Question as a new iphone user: how many times do you think a knock-off cable can be used before it can damage a phone, would you expect it to cause problems on the first time it's used or would it take a few times?

If it's bad and likely to damage the phone, it could do so the very first time it's used. It may also do so at any point after that.

Stick to MFI cables

Just to expand a bit on KiltedTim's post. It doesn't have to be a cable made by Apple. It just should be one that is certified by Apple. I've used cables by other reputable manufacturers for years.

Thankyou for the reply, I had to charge and was in a pinch so I used my old Pixel charger and cable. The phone didn't stop at 80% so I thought I damaged it but it was the first charge cycle.