How can we assign a datetime field value to "30 days from today" while we are creating any record through Flow?
Also how can we concatenate any two input values in flow if we want to use these two values as a combined value in a third field?
Your solution for both of these problems is to use the Assignment element.
To add days to a date, create a date variable, then create an Assignment element. Set your date variable in the first column, set the Operator to "add", and the Value column to the number of days you wish to add. This will add 30 days to the date in the variable, which you can then use in your Record Create.
To concatenate text, it works exactly the same way. Create a text variable, get your first block of text into it with the Assignment element, add a line in the same element with the same variable, but with the "add" operator, and the text you wish to add in the Value column. The Add operator, when applied to text, works just like concatenation.