The best articles for this are [1] (google translate it) and [2]. In [1], the author uses .asc files while in [2], .img files are used. You can treat these files as basically the same as .hgt but remember to reproject the data as different DEM sources uses slightly different projections.

[1] https://makina-corpus.com/sig-webmapping/representation-des-... [2] https://github.com/nst-guide/terrain