Salut,
la question est plutôt de savoir ce que tu appelle variable et ce que tu veux en faire 
Voici un example en utilise l’interface graphique de Node Red
Ceci est un Noeud qui récupère le niveau de batterie de mon téléphone.
Et en plus je créé un autre vairable de toutes pièces appelée « msg.blabla ».
Ensuite un peurt par exemple utiliser noued de type change pour crée le contenu de message d’une notification qui utilise les variables.
Le tout sotkcé dans une nouvelle variable appelée « msg.message »

{"message": "Batterie à " & msg.payload & "%","title": msg.blabla}
Et finalement utiliser la variable « msg.message » pour envoyer une notification sur mon téléphone.
Et voici la notif sur mon téléphone:
Le tout en version Json importable dans Node Red
[{"id":"c359d1f2f15c6aaf","type":"inject","z":"e747e193cdb9c0d8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":280,"y":880,"wires":[["bb4f8480e5d9caf3"]]},{"id":"bb4f8480e5d9caf3","type":"api-current-state","z":"e747e193cdb9c0d8","name":"","server":"ab763e9f.9a91b","version":3,"outputs":1,"halt_if":"","halt_if_type":"str","halt_if_compare":"is","entity_id":"sensor.pixel_4a_niveau_de_batterie","state_type":"str","blockInputOverrides":false,"outputProperties":[{"property":"payload","propertyType":"msg","value":"","valueType":"entityState"},{"property":"data","propertyType":"msg","value":"","valueType":"entity"},{"property":"blabla","propertyType":"msg","value":"ma variable blabla","valueType":"str"}],"for":"0","forType":"num","forUnits":"minutes","override_topic":false,"state_location":"payload","override_payload":"msg","entity_location":"data","override_data":"msg","x":590,"y":880,"wires":[["1a2116bc5b5bfb9b"]]},{"id":"584ff29fd538767f","type":"api-call-service","z":"e747e193cdb9c0d8","name":"Notification Monitoring Alex","server":"ab763e9f.9a91b","version":5,"debugenabled":true,"domain":"notify","service":"mobile_app_pixel_4a","areaId":[],"deviceId":[],"entityId":[],"data":"msg.message","dataType":"jsonata","mergeContext":"","mustacheAltTags":true,"outputProperties":[],"queue":"none","x":1240,"y":880,"wires":[[]]},{"id":"1a2116bc5b5bfb9b","type":"change","z":"e747e193cdb9c0d8","name":"","rules":[{"t":"set","p":"message","pt":"msg","to":"{\"message\": \"Batterie à \" & msg.payload & \"%\",\"title\": msg.blabla}","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":930,"y":880,"wires":[["584ff29fd538767f"]]},{"id":"ab763e9f.9a91b","type":"server","name":"Home Assistant","version":5,"addon":true,"rejectUnauthorizedCerts":true,"ha_boolean":"y|yes|true|on|home|open","connectionDelay":true,"cacheJson":true,"heartbeat":false,"heartbeatInterval":30,"areaSelector":"friendlyName","deviceSelector":"friendlyName","entitySelector":"friendlyName","statusSeparator":"at: ","statusYear":"hidden","statusMonth":"short","statusDay":"numeric","statusHourCycle":"h23","statusTimeFormat":"h:m","enableGlobalContextStore":true}]