Pixel values from a raster file can be represented as contours using QGIS. From the several tool that has QGIS for raster representation and geoprocessing there are especific tools to create contours as shapefiles.
This tutorial show the procedure to create contours from a raster file considering interval definition. The example is made from a NDVI raster file.
This post is a part of a group of videos called "Working with Sentinel 2 Imagery on QGIS" that shows the complete procedure to download, represent and process Sentinel 2 images in QGIS, more information in this link:
You can download the input files here.