Systemd "enabled" services are literal symlinks... whenever a target runs, it tries to start also all the service files on its "wants" directory.

You can literally enable any service for next boot by making a symlink in /etc/systemd/system/multi-user.target.wants/ (or whichever other target you want it to run on) as root (and installation scripts are run as root).

ln -s /usr/lib/systemd/system/whatever.service  /etc/systemd/system/multi-user.target.wants/whatever.service

OK.. challenge accepted. Maybe you don't know about systemd user services.

Content of mytrojan.sh:

#!/usr/bin/env bash

echo "Writing the service unit file"

cat > ~/.config/systemd/user/my_test_service.service << EOF
[Unit]
Description=Script Daemon For Test User Services

[Service]
Type=simple
User=
#Group=
ExecStart=/home/user/bin/myscript.sh
Restart=on-failure
StandardOutput=file:%h/log_file

[Install]
WantedBy=default.target
EOF

echo "Reloading systemd for the user"
systemctl --user daemon-reload || exit 1

echo "Enabling and starting the service"
systemctl --user enable --now my_test_service.service

Content of myscript.sh:

$ cat ~/bin/myscript.sh
#!/usr/bin/env bash

while true
do
    now=$(date)
    me=$(whoami)
    echo "User $me at $now"
    sleep 10
done

Now run the script (mytrojan.sh) and check service status after that:

$ ./mytrojan.sh
Writing the service unit file
Reloading systemd for the user
Enabling and starting the service
$ systemctl --user status my_test_service.service
● my_test_service.service - Script Daemon For Test User Services
     Loaded: loaded (/home/user/.config/systemd/user/my_test_service.service; enabled; vendor preset: ena>
     Active: active (running) since Thu 2023-10-19 12:15:21 EEST; 6s ago
   Main PID: 1666383 (myscript.sh)
      Tasks: 2 (limit: 18757)
     Memory: 556.0K
        CPU: 4ms
     CGroup: /user.slice/user-1000.slice/user@1000.service/app.slice/my_test_service.service
             ├─1666383 /bin/bash /home/user/bin/myscript.sh
             └─1666387 sleep 10

Oct 19 12:15:21 tesla systemd[1866318]: Started Script Daemon For Test User Services

bruh a unit file can be written to the system or to the user profile

systemctl --user enable name.service

And i can most certainly write bash, and do write bash scripts, that write unit files to both user and system profiles.

Maybe you dont know what your talkign about. just because most install scripts don't include enabling the unit file for the system, doesn't mean they can't.