Datasource named ${DS_PROMETHEUS} was not found error

I am reading the dashboard documentation and I have prepared a few dashboards.

The goal is to run the Grafana and preload those dashboards. I can accomplish this when the dashboard has in the templating list:

"error": null,
        "hide": 0,
        "includeAll": false,
        "label": "datasource",
        "multi": false,
        "name": "DS_PROMETHEUS",
        "options": [],
        "query": "prometheus",
        "refresh": 1,
        "regex": "",
        "skipUrlSync": false,
        "type": "datasource"

Some of the predefined Dashboards do not have those values. Since I a complete beginner on this I do not want accidentaly to mess up the files.

I searched the web and I found for example VictoriaMetrics / helm-charts and more specifically:

grafana.dashboards.default.nodeexporter.datasource	string	"VictoriaMetrics"

Based on this I expect that also Grafana has a key under providers maybe under options (key) where the user can define default DS?

The documentation does not describe something like that, and I can not find it anywhere online.

Can someone share more information on this?

Thanks in advance for your time and effort.

BR / Thanos