I am that male friend in the harem story - Chapter 12: My Senpai

Through a mysterious portal, Chen Jin arrived at this strange place – a post-apocalyptic world where civilization had been destroyed.

Consequently, he found –

A piece of metal scrap
A robot trash collector
A battleground
A city
A ton and more of gold
A copy of high-tech documents
A…
In short, Chen Jin found a planet.

- Description from Novelupdates

Hou Niao, devoted to seeking the Dao, found himself mistakenly sent as an undercover agent to the Demon Sword Sect, eagerly anticipating the day he’d be rewarded for his service and could return to the Daoist Sect… Only when he had become a towering figure in the Sword Dao did he sorrowfully realize that there was no one left with the power to recall him!

The story of otaku building a harem! School flower, loli, wife, mother and daughter, mature woman, teacher, nurse, queen, all kinds of beauties, three steps to push a girl, five steps to a group, harem group, infinite warmth, so cool that you always need to prepare a tissue for easy reading !

- Description from novelbuddy