Where software lives

When you open Spotify on your phone, where is Spotify?

That is not a trick question. It is genuinely one of the most useful questions in this course, and roughly 80% of the confusion you have ever felt about modern software dissolves the moment you can answer it cleanly.

Take ten seconds. Where would you say it is?

OK, here is the answer in two words.

Local. Some software runs on the actual physical device in front of you, the thing in your hand or on your desk. If you yanked your device's Wi-Fi cable, the software would keep working.

Remote. Some software runs on a different physical device that lives somewhere else (often a building you have never seen, full of fans and air conditioning), and your device just talks to it over the internet. Yank the Wi-Fi, the software stops working.

Most modern apps are a little bit of both, where your phone does some local work to render the screen, and a server somewhere does some remote work to fetch your email or your songs or your prompts. That hybrid is so common that it is the default assumption when someone says "an app" in 2026.

That is the whole vocabulary for today. Two words, local and remote. You are done.

Why we are going to sit on this for ten minutes

Because once you see it, you cannot unsee it, and almost every question an AI tool will ever ask you depends on which side of this line your thing is on.

When Claude asks you do you want this hosted as a serverless function?, it is asking you a question specifically about the remote side: it has assumed you've already decided the answer is "remote, not local," and now it just needs to know what flavor of remote.

When Cursor asks you should this run on your machine or in CI?, it is asking which device should do the work, your local laptop or a remote machine that runs every time you push code.

When Notion asks if you want a local cache of your workspace, it is asking whether to keep a copy of your remote data on your local device, so it still works on the subway.

Each of those is the same question wearing a different costume.

The Wi-Fi test, which is the only test you need today

Pick any app on your phone. Now imagine your Wi-Fi disappears for an hour, with cellular off too, pure airplane mode.

Which apps still work?

Calculator. Yes. Local.

Notes (the one Apple ships). Mostly yes. Local, with some quiet remote syncing in the background when you are online.

Slack. Sort of. You can read the last few messages, but you can't send anything new and you can't load anything fresh. Mostly remote.

ChatGPT. No. Pure remote. The model lives on OpenAI's servers, and your phone is just a window into it.

The camera app. Yes. Pure local. Photos taken in airplane mode are taken on your device and stored on your device. Sharing them later is a separate, remote concern.

Maps. Tricky. Map tiles you have already loaded will still display. Anything new will not. So half-local, mostly-remote.

If you want the cheapest exercise this course will ever give you, run the Wi-Fi test on five apps you have open right now, and write down your answers. That is Day 3 homework, it takes two minutes, and it pays off for the rest of the course.

The "both" case, and why it confuses everyone

Most useful software in 2026 is both local and remote at the same time. Slack runs a local app on your phone to render messages, sync to disk, and let you scroll smoothly. It also relies on a remote service at Slack's data center to actually receive new messages, store them, and broadcast them to your colleagues. Neither half of the system is "Slack." Slack is both halves, talking to each other.

This is what trips up almost every beginner. People have an instinct that there is one place where the thing "really" lives, and they want to know which place that is. The truthful answer is usually "it is split across two places on purpose," and Day 4 is going to take that split apart in detail.

A small note on dialect

You will hear other builders use other words for these same two ideas, and the words mostly mean what you would expect.

• Client is usually a synonym for the local thing (your phone, your browser).
• Server is usually a synonym for the remote thing.
• Frontend and backend are the names for the same split when we are specifically talking about a website (Day 8 and Day 9).
• Serverless is a marketing term for a particular kind of remote, where you don't have to think about which specific computer your code is on. The name, as promised yesterday, is a lie. There are servers, you just don't have to care which.

When someone uses one of these words, mentally translate it to "local" or "remote." That is enough vocabulary for the next four days.

Forward references

On Day 4 we follow a single tap from your phone, across the internet, to a remote computer, and back. On Day 11 we separate where your data lives from where your code lives, which is a frequently underestimated distinction. On Day 20 we come back to remote is never neutral, because the moment your software lives on someone else's computer, you have obligations you do not have on your own laptop.

Day 3 wrap

The thing you can now say plainly. Software either runs on the device in front of you (local) or on a different device somewhere else (remote), and most modern apps are some mix of the two.

The thing you can now do. Run the Wi-Fi test on any app and decide on the spot whether it is mostly local, mostly remote, or half-and-half.

The guardrail to remember. Remote is never neutral. The moment your code runs on a computer you do not own, you have to think about cost, privacy, reliability, and trust in ways that local code never asked of you.

See you on Day 4, where one tap goes on a longer journey than you would think.