Oh man, this one.
Jimbocho in Kanda, Chiyoda ward (神田神保町) is a book nerd paradise - it’s between multiple universities and has lots of used bookstores. Kanda is also literally across the river from Akihabara, so this is prime nerd territory.
The first place that Oosara’s brother and his friend lined up on is Jiro (二郎) and it’s a fairly well known chain.
The second place…I am not so sure about (I can’t find a Tsukemen place in the area with a similar sign and the area seems to have more abura soba joints), but the third (where she ate the porky bowl and ran into Koizumi-san right afterwards) was 肉そば総本山神保町けいすけ Nikusoba Sohonzan Jimbocho Keisuke, or roughly Keisuke’s Jimbocho Meat Soba shrine.
Unfortunately it no longer exists…as it shut down back in May 2023. Don’t feel too sad though -
Keisuke’s a mini chain with locations not too far away in Ginza and Kinshicho (featuring less pompous names), and while they only have 4 stores in Japan (18 if you count the other restaurants in the Keisuke culinary empire), they have locations overseas in Singapore (17 for now).