Node.js 为什么我的 Heroku Express API 数据是持久的，即使它只是来自一个变量

我使用express创建了一个简单的API，并将其部署到Heroku，代码如下:

const express = require("express");
const app = express();
const cors = require("cors");
app.use(express.json());
app.use(cors());
app.use(express.static("build"));
let notes = [
    {
        id: 1,
        content: "HTML is easy",
        date: "2022-05-30T17:30:31.098Z",
        important: true,
    },
    {
        id: 2,
        content: "Browser can execute only Javascript",
        date: "2022-05-30T18:39:34.091Z",
        important: false,
    },
    {
        id: 3,
        content: "GET and POST are the most important methods of HTTP protocol",
        date: "2022-05-30T19:20:14.298Z",
        important: true,
    },
];

const generateId = (arr) => {
    const maxId = arr.length < 0 ? 0 : Math.max(...arr.map((item) => item.id));
    return maxId + 1;
};

app.get("/", (req, res) => {
    res.send(`<h1>Hello World!</h1>`);
});

app.get("/api/notes", (req, res) => {
    res.json(notes);
});

app.get("/api/notes/:id", (req, res) => {
    const id = Number(req.params.id);
    const note = notes.find((note) => note.id === id);
    if (note) {
        res.json(note);
    } else {
        res.status(404).end();
    }
});

app.delete("/api/notes/:id", (req, res) => {
    const { id } = Number(req.params);
    notes = notes.filter((note) => note.id !== id);
    res.status(204).end();
});

app.post("/api/notes", (req, res) => {
    const body = req.body;
    if (!body.content) {
        return res.status(400).json({
            error: "Content Missing",
        });
    }
    const note = {
        content: body.content,
        important: body.important || false,
        date: new Date(),
        id: generateId(notes),
    };
    notes = notes.concat(note);
    res.json(note);
});

app.put("/api/notes/:id", (req, res) => {
    const newNote = req.body;
    notes = notes.map((note) => (note.id !== newNote.id ? note : newNote));
    res.json(newNote);
});

const PORT = process.env.PORT || 3001;
app.listen(PORT, () => {
    console.log(`Server running on port ${PORT}`);
});

as you can see, the data served to the frontend (A React app) comes from the '/api/notes' endpoint, this endpoint returns a response with the notes array.
After deploying to Heroku (https://fierce-chamber-07494.herokuapp.com/) the functionality of adding notes, and setting importance all work perfectly normal, but what I wasn't expecting was for the data to be persistent even after refreshing the page, visiting it in another device, etc. The data only comes from a variable, not a database, nothing. So why is it persistent? does Heroku modify the variable itself? how does this work? I hope my question is clear.