Or, if they're in a reasonably dense area, people go out to a nearby restaurant. Bring your own lunch has almost certainly declined over time. During the 10 years or so I worked for smaller companies in cities, it seemed as if going out for lunch with coworkers or otherwise was pretty much the norm. (Otherwise there were company cafeterias at the larger firms.)
When I worked downtown a number of years back a lunch special in Chinatown was about $6 and a nice break in the middle of the day. Not sure there was even a microwave in the office. Another company did have a lunch room but there were a number of good soup and/or sandwich places within a few minutes walk. Meals in any case were pretty similar to what I’d make at home and maybe a few dollars more expensive.
Boston. Chinatown was maybe a 10 minute walk away. Other "downtown" location I worked was Nashua which was maybe 45 minutes north. No Chinatown but a bunch of small soup/sandwich places. Other (bigger) places I worked were suburban/exurban and there were very limited convenient options other than within the company.
Boston is tops for walkability. I think people don't appreciate this if they're not familiar with Boston but you can walk from one side to the other of downtown Boston in 30 mins and pass a half dozen T stations on the way.
Can be, but e.g. in London all the major office space hubs also have a bunch of places that cater to office worker lunches. They tend to be comparatively cheap (emphasis on “comparatively”, mind you), and healthier than restaurant food.
When I was in London, I saw something that I don't recall seeing anywhere else: small (but much larger than a 7/11 type store) supermarkets selling packaged food and they had a dining area (tables, chairs, cutlery) after the checkout. I think they even had microwaves.
Often smallish but a big cut above US convenience stores such as M&S/Sainsbury in the UK are pretty common in Europe generally. I still wouldn't be inclined to use them for lunch sandwiches in general but they have better than the slop you'll get at a US 7/11. I can't say I've really observed the dining area in general outside of some of the big food halls. (There are food markets with such but those exist in some big US cities as well.)
I think their lunch sandwiches are perfectly acceptable, I used to eat a Sainsbury's meal deal every day. Anecdotally I find the UK's prepackaged supermarket sandwiches are much nicer than than the equivalents on the continent.
Most smaller places it is “bring your food” or they get food in from a caterer to set up in the break room.