First of all:
- Just download the 1"-sec versions of the DTMs (or if you have little available memory on your phone: 3"-sec).
Not the 20m and 50m-versions, Locus can not use them.
- Yes, copy the .hgt-files into data/srtm folder of Locus.
- If there are already equal named files in this directory, just overwrite them (they have been automatically downloaded by Locus before and are based on less accurate SRTM-data)
You don't have to copy each .hgt-file of a country, just for the areas you are interested in.
Each elevation file (.hgt) represends exactly 1° x 1°, which could automatically derived from the filename. The filename specifies the southwest corner, e.g. "N47E014.hgt" represends an area from N47° to N48° latitude and E14° to E15° longitude.
- If you copied them I recommend to start Locus and once execute
Menu > Settings > Miscellaneous > Clear temporary data > (check all points)
to make sure, no old cached elevation or shading remain.