Is there a trick to threading string back into

Pin a safety pin to the end of the string to help you pull it through.

Yes. You fasten one to a safety pin and use that to guide it through through the fabric.

An old-style metal clothes hanger, snip off the hook with wire snips or a fancy pliers, straighten the rod, bend a little hook at one end, thread it through the pants, attach/tie the cord to the bent end, pull back through. You can always throw a little tape around the bent end to make sure there's nothing sharp to catch when you pull it back through.

They sell things to help thread them through but a safety pin should work, it just takes time to work it through

Tie one end to a knitting needle and thread it back around 🤷‍♂️

That skill has been lost by humans

I’ve never been able to successfully rethread my sweatpants or hoodie