I found from Forum an explanation to why File Manager does not give ‘install’ option.
The apk file is interpreted as a zip file, like below.
[root@XperiaXA2-DualSIM defaultuser]# xdg-mime query filetype /home/defaultuser/Downloads/Signal-Android-website-prod-universal-release-5.30.6.apk
application/zip
[root@XperiaXA2-DualSIM defaultuser]#
The bug is fixed in 4.1.0 , so I guess I need to update to that one first… dang… it was nerve-wracking the last time I did SFOS update, and I feel the same now already…