TRIGSTR_XXXX are always from the war3map.wts file, I can't really tell if its constant because. If you removev TRIGSTR_XXXX and replace it with a normal text, the map COULD still work fine. Although, it's much safer to change it from the war3map.wts because it's the way how the map was made.
Continuing the same topic, kinda, If i know that hero + item triggers channge of the hero, or item + item triggers to a new item where can i find these triggers ?
you can search the rawcode of the item in the war3map.j file and try to piece it together
If you have any questions drop in by chat sometime, chances are there'll be someone who can help you that's afking there, so the next best thing is to click the link on UndeadxAssassin's Sig and ask your question there.
go back to where it registers the event
it should be like triggerregisterenter then region or regionsimple or rectsimple
find that and see what region/rect it points to
get a deprotected copy of the map
find the coordinates
enter that place ingame
voila