My own experience ([1]): the biggest different between "startup inside BigCo" and an actual startup is availability of resources.
"Startup inside BigCo" generally revolves around spinning up a new team that's product focused and delivering quickly.
Depending on which BigCo you're at, delivering quickly could be a departure from how things are normally done. e.g., You don't have to worry about writing a big up front proposal doc, going to the architectural review board, or using the standard infra tooling.
For engineers on a team, this "feels" like a startup. There are daily standups. They talk to users and have a sense of ownership on what's built. There can even be some sense of urgency to ship quickly.
While "BigCo startup" teams mimic a lot of the same procedures and activities as a startup, there is an underlying support structure at BigCo that isn't there in an actual startup. IMO, that makes the experiences substantially different.
Some examples:
- The cost of failure is different. At BigCo, if the project fails, it's generally OK. Each engineer can be reassigned to some new team at BigCo. At a startup, if the project fails, the entire company fails.
- There are more people to ask for help. At BigCo, if you're stuck on some hairy engineering issue, you have a swell of engineering talent to lean on for help and guidance. At a startup, you have StackOverflow, Google, and maybe some folks in your personal network to lean on for help.
- There's more than just product development. At BigCo, generally only the engineering / product development team is structured like a startup. After the code is "done", the regular product marketing org, marketing org, and sales org (if that's a thing), can kick into gear. At a startup, there is no such massive support structure.
- On the topic of marketing, having the brand of BigCo is a huge boon to a new product. For mobile apps, there's a lot more trust in seeing "NewProduct by BigCo" vs. just "NewProduct" in the app store. Your rockstar CEO may even tweet out the launch announcement to his millions of followers.
- And yes, at BigCo, when it's lunchtime, you can go to the cafeteria and decide if you want the steak or the coq au vin for lunch. (well, before COVID at least)
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[1] - I've worked for really small startups (< 8 people), really big companies (I was person 2000+), and small companies that became big (joined at 20, left at 200+, now at 500+).