replying to a thread in another thread?

how can it be done?

Copy and paste the URL to the original post. 4chan image boards support >> anchors that cross boards, but world4ch doesn't. Yet.

all the threads have random numbers in the links instead of numbers starting from 1 which is much nicer

The numbers are the date the thread was started, like how image names are the date the image was posted. Global post ids aren't done on 2ch and are only an accident of using SQL on 4chan.