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By moving your code to getValues, you also changed the scope in which your temp variable exists in. Variables are not automatically globally available. If you declare a variable inside a function (which getValues is), it's only available in this function.

When you try to access the temp variable in your loop function, the compiler rightly tells you, that there is no such variable available.

You could solve the problem by declaring temp as a global variable, which you would do by adding float temp = 0 up on top where you also declare variables like soilMoistureValue. Make sure not to redeclare the variable in getValues then, so instead of declaring like so float temp = dht.readTemperature(); you just assign a new value like so temp = dht.readTemperature();

A quick note on your first paragraph: The voids and bools how you call it, define the return type of a function. If your function does not return anything, you define it as void. If it returns a boolean value (so true or false), you define so bool. In the case of your getValues function, since it does not return anything, it should be void getValues.