If you mean that you're reading text from a file using the Read text from file action and you expect that a variable notation in the text will be automatically replaced with the variable value, that is not going to happen. This is not how it works at all.
When you read text from file, it is read literally. The flow will assume your string says "Hello %var%" and will not understand that you want %var% in the string to be interpreted as an actual variable.
A better approach would be to read the text from the file and then use Replace text on it to replace the placeholders in the text with variable values. If the placeholder is not found, nothing will be replaced, so this can be used dynamically.
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