Should we reinsert trend after doing forecast using detrended data? Basically, when we detrend a signal, we detect and remove a linear component of that signal. This produces a stationary version of that signal. And we can use various forecasting algorithms to forecast if the signal is stationary.
I checked a fair amount of notebooks/tutorials and I noticed that all forecasting tutorials finished after making forecasts from detrended data. I want to understand what I am missing or maybe these tutorials leave out the final (reinserting the trend) part since it is simple.

Since you presumably want forecasts for the original (trended) series, yes, you should insert the extrapolated trend after forecasting the detrended series. "Insert" here can be additively or multiplicatively, depending on how you detrended the series in the first place. And note that in extrapolating trends, it is often useful to dampen it, rather than let the trend grow without bounds in either the positive or the negative direction.
