Why don't pre amps have that built in auto function general recorders have? Wouldn't it make things ten times easier?
Auto level setting is similar to a limiter. Both monitor the input level to the device and adjust the output level. In the case of a recorder, the output level is the recorded level. Both can be useful as a "crutch" to help get a usable recording if you are unable to set the recording level correctly, if the sound source being recorded is too unpredictable.
Auto-record level constantly adjusts the recording level, which can be useful for recording lectures or speech where the intent of the recording is transcription without needing to pay too much attention to the recording, rather than natural and pleasing sounding audio quality. It is not a good choice for recording music, where it will be constantly trying to make the louder parts quieter and the quiet parts louder, but cannot do so very smoothly. You will hear it working, like someone constantly turning the volume up and down, clumsily. If you set your recording level correctly so that there is enough room to fit the music, you don't need auto-level setting.
A limiter on a preamp or recorder can be useful for music recording if the recording level has been set high enough that the signal would overload and distort badly during quick (transient), high level peaks. A limiter is not designed to be constantly engaged, it should only "do it's thing" momentarily, and then release quickly. If it is only working occasionally, its effect will not be as audibly apparent as the auto-level setting function. But if it is working at all, it is reducing the full natural range of the music. If you set the recording level low enough so that there is enough room for the peaks without overloading, you don't need a limiter.
You can make better recordings by setting your recording level appropriately so that you do not need to use either of these things on the recorder.
Do I use the limiter on the unit, or the limiter on the software app( like the tascam pcm Rec mkII). Or both ? I record loud metal so it needs a limiter for sure.
Limiting after the recording has been made serves an entirely different function. It is used to increase the apparent loudness of the recording beyond what one could do by simply turning up the signal level until it is just below overload. There is no need to do that to prevent distortion. Doing so actually introduces distortion.
You won't need a limiter "for sure". A limiter isn't useful because the sound being recorded is loud. You can simply turn down the recording level to fully accommodate "loud". A limiter is useful when the overall range of loudness is very large and hard to predict. With loud metal, the average level is very loud, yet the overall range of loudness during the music is not very large- the peak levels aren't going to be a whole lot higher than the average loudness level. It gets loud and stays loud. It doesn't have very rapid, very high peaks that are considerably louder than most of the music.
Set your levels so that you can accommodate the highest peaks without overload and leave some safety margin. You probably won't notice any low-level noise introduced by your equipment during the quiet parts. They aren't going to be quiet enough to hear the noise.
Also lets say I put the level too low on the pre amp. Does it really matter since the recording levels on the recorder, in this case the iPhone , will be set correctly and a little higher since the pre amp is too low. Is it something you will really notice ?
That's "gain-staging", and really what this thread is all about. It's all about setting the relative levels through each device correctly so that everything works as best it can- neither causing distortion of the signal nor introducing noise. It matters if the gain-staging is set inappropriately and causes either of those problems. It doesn't matter if neither of those problems are introduced. There is usually a range of settings which will not introduce either of those problems, and they only become a problem when your gain-staging falls outside of that acceptable range. Setting optimal gain-staging makes the breadth of that range as wide as possible.
Once you have a raw recording which is neither clipped at the top (from a level being set too high somewhere) or buried in noise (from a level being set too low somewhere), you can adjust the level and "loudness" afterwards with editing software however you like. Doing that is the topic for another thread.