that is a quite popular misconception. Although the DEMs delivered with the ALOS RTC files have a pixel size of 12.5 meters, they are mostly just up-sampled DEMs from SRTM to match the spatial resolution of the input data (ALOS PALSAR with 12.5 meters in this case)
Source : https://www.asf.alaska.edu/asf-tutorials/dem-information/
So the quality is not better than any SRTM which is automatically downloded by SNAP (SRTM 1 ArcSec). You can use it right away.
If the quality is not sufficient, you can also manually download ALOS W3D30 data here:
https://www.eorc.jaxa.jp/ALOS/en/aw3d30/index.htm
Their spatial resolution is also 30 meters but it was originally generated from stereoscopy at 5 meters spatial resolution, so in many regions it contains finer details than SRTM. I made a comparison once which shows it a bit.
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