It's based on a weekly series of illustrations of buxom girls. The artist (Kiseki Himura) has been posting one every Monday since 2015. This got a series of internet episodes in 2016, a sequel season in 2021 and a manga series in 2020. (Apparently, the latter has several characters who don't appear in the anime.)
It's quite nice. I enjoyed it.
It's about some girls with big boobs and their faceless male friends. Almost everyone in this show is faceless, with no eyes or noses. Occasionally they don't even have a voice. (Sensei tends to "talk" in gestures and we never hear his words, although he sometimes makes audible noises.) They also don't have names, with everyone being called Junior, Teacher, etc. You might think that Ai-chan is called Ai, but that's actually a reference to her I-cup breasts.
As a result, it can often be hard to tell who's who. I've just read on the internet that Senpai and Salaryman aren't the same man, but I watched the whole series without realising that. (They have different voice actors, but they're easily confused since they have the same defining feature, i.e. "works too hard at his ridiculously demanding office job".) Even the girls are hard to distinguish, since they all have the same generically cute anime face. Kouhai has a big old forehead and Maegami has her bangs hanging down to cover her eyes, but good luck keeping track of everyone else. You'll usually be guessing from context, i.e. Senpai's the one who talks to Kouhai, etc.
That said, though, it's a likeable show. It's more wholesome than you'd think from its origins and all the relationships are pleasant. It's meant to be soothing. It is.
2016 SEASON (12 episodes + 2 OVAs)
This is the simpler of the two seasons. Its only pairings are Salaryman/Ai and Senpai/Kouhai, although there are supporting characters like Volleyball-chan (who thinks being female gives her infinite licence to grope Ai-chan's boobs in public) and Senpai and Kouhai's boss (who's male and yet has a face).
(a) Salaryman has a tough job and regularly has to work overtime. Ai-chan is a schoolgirl who travels on the same train as him every Monday morning and cheers him up. That's it, although after a while they're going on trips and dates together.
(b) Senpai and Kouhai work at the same company and their boss is shocking, even if you're familiar with Japanese work habits. He might phone up Salaryman after going-home time and order back to the office to complete a bunch of additional work before tomorrow morning. (Salaryman should sell his mobile phone. It might create some inconvenience in his day-to-day life, but at least it would restrict his boss's communications to work hours. That's why they're called work hours.) The boss also thinks nothing of making everyone work over the weekend to hit a deadline.
This season is just a good-natured slice of life show. Nothing goes anywhere and nothing much happens. Ai-chan's school makes her run a marathon. Salaryman visits her at her part-time job. That kind of thing. The camera loves staring at girls' chests and it's mildly eye-rolling, but there's no nudity or sex. The second OVA (ep.14) is mildly naughty, though.
2021 SEASON (12 episodes + 1 OVA)
It's basically the same, but more ambitious. It has slightly longer episodes and a bigger cast, including:
(c) Maegami is at high school and she's in love with Sensei. She asks him out, but he says "no". For three years, she doesn't give up. He keeps saying "no". Even on her graduation day, she's still pestering him and he's telling her to take a hike. Later that day, though, she appears unannounced at his apartment in the rain... and when midnight comes, she'll no longer be a schoolgirl.
That's just their first episode, by the way. They move in together, get married and have a wedding (in that order). It's a romance, in other words, which is something the 2016 stories never did. Those were a tease. Oh, and Maegami can be funny.
(d) Cheerleader is a high school cheerleader who got caught by TV cameras and is now the hot thing of the moment. She's appearing on TV, doing bikini photoshoots, etc. She has a male classmate who's her childhood friend. They're quite fun, but they only get a couple of episodes.
(e) the non-heterosexuals, although they're not a couple yet. It's only a maybe-possibly. Ai's boob-groping friend, Volleyball-chan, has been getting ever more overt in her appreciation of Ai and her assets... and Ai's similarly endowed little sister appears to have a thing for Volleyball-chan.
It's a likeable show, with honest, healthy relationships... but it's still a silly fantasy. It's portraying a world where "Insert Your Face Here (Literally)" protagonists don't have to do anything to gain the affections of a devoted girl with huge boobs. Mind you, breasts are almost universally huge in this universe. It doesn't matter where you look. Air stewardesses, gym instructors, dentists, nurses, mothers, petrol pump attendants, you name it. Also, Volleyball-chan turns into a parody of herself in the 2021 OVA.
I enjoyed it. But it's fluff and a showcase for big boobs.