BL Isekai

Kondou Seiichirou is transported to another world when he tries to save a young girl from being sucked into a portal. Whilst there, he learns that the girl was a 'saint' meant to save the world and he was just an unfortunate accident. Refusing to just sit around, he decides to work in the palace accounting department, and learns just how bad their financial situation was. Turns out they needed him more than they anticipated.

Seo Jaewoo wakes up to find himself in the world of a BL novel. He figures out that the character he now inhabits is the villain and takes steps to smoothe things over with the main protagonist. They always said the key to a man's heart is through his stomach, so he regularly cooks for the protagonist and his little brother. In doing so, he slowly embeds himself into their life.

When Zhang Li left for a trip to the Ebiz ruins, he didn’t expect to bring back company, especially not a man made of stone! Though Li grows fond of the living statue, their time together doesn’t last long, as “Rocky” soon crumbles before Li’s eyes. Wishing for a miracle, Li suddenly finds himself in Ebiz far back in the past and reunites with Rocky. However, his companion is no longer a statue but rather the nation’s king

One day, without warning, I woke up as Seo Seungwon, a character in a BL novel given to me by my younger sister. The aftermath of being transported into this world is brutal—I’m plagued with relentless nosebleeds, bouts of coughing up blood, and unimaginable pain. I discover that staying calm lessens the torment, so I desperately try to empty my mind and maintain my composure.

Sacher Blythe, desperate to live a long and peaceful life, fails to avoid the future demon king, Phelan Exa Drozen. But Phelan knows more than he should…he recognizes Sacher as an “outsider”–someone from another world who knows the future and wants to kill him. So Phelan proposes a truce and Sacher agrees, hopefully preventing him from becoming the demon king. But this agreement keeps Phelan closer than Sacher would like…

Minjoon, a normal office worker, wakes up inside the game he was reviewing for his friend. It's not his fault the trailer was so boring it put him to sleep! Bewildered, Minjoon is tasked with summoning a hero to guide. The hero certainly looks strong, but he seems to be less useful than expected.

Jaewon, a small-time DIY trader, wins the lottery, but wakes up inside a web novel after a car crash. To return home to his prize money, he must help the male and female leads get their happily ever after, but why do all the handsome male lead candidates seem more interested in him than the heroine?

Huimin Seo happened to read a tragic BL novel where the two main characters had both his name and even his best friend’s name, Iheon Cha. But rather than remaining as an incredible coincidence, this book becomes a new reality for Huimin. Before he knows it, he is transmigrated right into a major scene in the novel where Huimin Seo, a rare dominant omega, gets bought off in an auction by Iheon Cha, a powerful dominant alpha. Huimin must tread carefully to turn things around entirely.

The main character is a chill guy that has always lived with the unfortunate stigma of looking exactly like a crazy villain character from a specific webtoon. One day he is suddenly transported inside of that webtoon and seizes the opportunity to turn his life around.
(I really like this one)

The main character is transported to another world and now inhabits the body of a bird.