Beginner Level Question: How does the Uno run code independently?

Using the Arduino Development Environment I can upload a sketch, or code to run on the Uno. According to the Reference guide I am using the Arduino boot-loader, this allows me to upload code without using any additional hardware. The boot-loader is active for a few seconds when the board resets; then it starts whichever sketch was most recently uploaded to the micro-controller.

This means of course I cannot save the last code uploaded to the Uno without more knowledge.

Let's say I have developed some code to interact with my environment, I upload the sketch say "alarm controller" and it runs looking at it's inputs until a breach occurs then does something, say sounds an alarm.

Fine, each time I want to run it I have to upload it via the Arduino Development Environment?

Where do I look for further instructions? Knowing the right questions is half the answer.

Many thanks

You load the compiled sketch and it is saved in flash memory. Next time you power up or reset, the same compiled sketch runs again.

thank you