1) For scan a QR code, which app do I need?

1) How to scan a QR code?

You should use the search function on the site where you get apps for your device.

I don't need any app on my phone as it came included in one of the android upgrades.

I just have to turn on the  camera ,point it at the QR code and the website connected to the QR code pops up on the screen.

I have no idea if this procedure works differently with non android phones.

In Android 5 and below, you need to install a 3rd party app. Pick up one from the Google Play Store.

In Android 6 and 7 you can enable "Now On Tap". If you don't want to, or in your country it is not available, install the 3rd party app like above. 

In Android 8 and later, QR code reader is built-in a camera app, but you might have to enable it in Settings.

Different device manufacturers have different camera apps, so search the web for "how to enable qr codes " plus your device model name and OS version.

Edited September 27, 20214 yr by Sensei

8 hours ago, PeterBushMan said:

1) For scan a QR code, which app do I need?

1) How to scan a QR code?

It depends on what the code represent. As said above many phones can recognise a QR code by using the native camera app. Interpreting the QR code and then triggering the correct action may require scanning the QR code with the applicable 3rd party application. 

My phone is very old.

If you want to write your own program for reading QR codes, for example in C#, you can use the ZXing library

30 minutes ago, PeterBushMan said:

My phone is very old.

Ok. Very old phones have no camera. So a newer phone may be required to solve your issue.

(You may get more helpful responses if you provide more context)

On 9/30/2021 at 11:28 AM, PeterBushMan said:

My phone is very old.

Too ambiguous.

"Very old" for most modern people are devices of 4 years or more.

Give the manufacturer and model name, operating system version, etc. etc.

On 9/30/2021 at 10:28 AM, PeterBushMan said:

My phone is very old.

Then it is unlikely to be able to run many of today's apps in any case.

This is certainly the case for my 7 yr old G3 iPhone. It does have a camera but it is full of dust inside and takes terrible pictures, so it would have no chance of scanning a QR code, even if it could run the app.