Recording bass question....

cant speak for all, but there are external mics for the iPhone/iPad that are significantly better than the built in mic on those devices. You can get quite decent recordings using these improved external mics and an EQ app.
for example (but there are many)
Zoom iQ6 Stereo X/Y Microphone for iOS Devices with Lightning Connector

 

I often use an apogee for recording to my computer. I don't have an iPhone, but there is an attachment for the iPhone. It lets you use any mic you like. I use it for recording acoustic guitar. I'm not sure for the phone, but the computer let's you use it as a ghost power source for the microphone if needed.

 

Regular iPhone mics are better than most people would expect. The clip on mics are better. In either case it sounds like you don't have your levels right. Crank it up so it has a stronger signal on recording.

For the iPhone internal mics You need to use a music app to record. GarageBand on a iPhone is a great starting point. Watch the levels in GB.

If you need a better mic setup like one on your bass and one on your voice then you can get an IOS compatible interface.