how can i use for travel overseas

Most overseas locations have had physical pre-paid nanoSIMs readily available for decades.

Many of these same carriers now can also provide prepaid service via an eSIM.

Certainly the easiest way would be to simply find such a carrier, scan their QR code, and - after some “configuring” on your iPhone keyboard - voilà - you have a 2nd “overseas” line on your phone.

If you already use an eSIM for your primary line, then you then have the additional option of using a nanoSIM while travelling; something very useful if a particular regional carrier doesn’t provide an eSIM option.

Note that you phone can only have a single active CDMA line at any one time.

And your phone MUST not be “carrier locked” to your current carrier.

Lastly, unless you are using a Chinese-market iPhone, you have only a single nanoSIM slot and an eSIM (which can hold multiple carrier profiles).

The article linked below is the more appropriate reference for iPhones in “non-China” markets:

Using Dual SIM with an eSIM - Apple Support