I'm making a post apocalyptic story that take place 20,000 years in the future. And I want to know if modern civilization ruins would be around after that. stuff like buildings, signs, transportation Etc would that stuff be around 10,000 to 20,000 years? I know buildings such as the great pyramids, or the great wall of china would be around, but would modern buildings be around?
10,000-20,000 years means at least one ice age.
That means any cities in the ice and glacier zones are completely wiped clean as the landscape is resculpted.
That also means significant sea level changes, so most port cities either underwater...or resurfaced but thickly buried in sediment.
As long as your cities aren't in those two zones, concrete and stone ruins seem likely to survive.