I never used Altium myself.
I heard some complaints about Altium that it thinks it’s king and changes lots of file associations to link to itself. Even C source code files and configuration files for other IDE’s.
How did you make that Test.sch file?
A size of 72 bytes is suspiciously small.
It’s contents are:
EESchema Schematic File Version 2
EELAYER 25 0
EELAYER END
$EndSCHEMATC
If I then open it with Eeschema V5.1.8 it opens as a normal empthe sheet, and if I then save it, it grows to 208 bytes and it’s contents are:
EESchema Schematic File Version 4
EELAYER 30 0
EELAYER END
$Descr A4 11693 8268
encoding utf-8
Sheet 1 1
Title ""
Date ""
Rev ""
Comp ""
Comment1 ""
Comment2 ""
Comment3 ""
Comment4 ""
$EndDescr
$EndSCHEMATC
That first file must be from a very old Kicad if the file version changes from 2 to 4. (V5.99, soon to become V6 has a completely different file format based on S-expressions just as Pcbnew)
Double clicking on a schematic file is also not a good way to work with KiCad, as it opens Eeschema (if altium does not interfere) in “standalone” mode, and this is not fit for working with a KiCad project.
When working with KiCad, you should always start with the KiCad project manager, which looks like:
And then, in the project manager, create a project with:
File / New Project, and then start Eeschema, Pcbnew and other sub programs by clicking on their icons.
More about KiCad projects in: