For professional work, there is no substitute for a quiet and substantially echo free space for recording. There’s good reason why professionals still use recording studios.
For amateur work, you may be able to achieve good enough results using a directional external microphone rather than the smartphone’s built-in microphone, and/or applying lots of small edits to cut out the worst bits. Filtering and other noise removal effects are likely to fail because car horns produce a lot of harmonics across the vocal frequency range, so such approaches will cause a lot of damage to the sound quality of the voice.
