Averaging some layers

I am working with Sentinel 5p data. I reprojected the Daily NO2 column density product and extracted them as a single layer. Now, I want to average them and produce monthly NO2 products for my study area. Please let me know how I can do in SNAP. I tried “band math” but it is not useful for me.

The right tool for you is the L3 Binning. There you can define your inputs and create aggregated results.
See in the menu Raster / Geometric / Level-3 Binning it is also available as an operator on the command line.

Unrelated to the sentinel toolbox but the harp can be used to process sentinel 5-p in python to generate monthly average maps. It is fast, automated and can be repeated over for any number of months. The only thing you need to give is bounding lat, long and date. The code will download the data and produce average NO2 map + weight map(you have to provide a quality flag for clouds and hence all the images are not used for NO2 average purpose, the weight map gives number of images used).
The code can be found at https://github.com/der-knight/sentinel5p-NO2/blob/main/sentinel_5p_average.py
The blog on how to do this processing is at https://nirdeshthekumar.medium.com/monitoring-air-pollution-with-satellite-data-using-sentinel-5-p-in-python-2bbc6e1acef4
and if you use the code the paper citation is at https://doi.org/10.5281/zenodo.4569495