I would like to start shooting videos with my smartphone. Is there a free video editing software for Android phones that does remove the audio and doesn't add watermarks? Or do I have to transfer the video files to my laptop for editing?

This site is considered to be extremely informative, but I would like to know if this data is deleted and why.

Adobe Clip can mute audio or replace it with something else. But exports max 1080p. Other options is to turn audio recording off with camera app like Filmic Pro for example.
But neither are free.

Try to use and evaluate the free (watermarked) version of KineMaster.
It goes by the name KineMaster � Pro Video Editor at Google Play.
At my Samsung S9 I can copy with an OTG USB my Sony 4K 100 Mbps files, edit audio & video, add a second and a third title layer and export in 4K at a bitrate not close to the original but still at about 50 Mbps. The trimmed project took me 50 % more than its total duration time to export.
So, it is many times faster than an old laptop.
Yes, the screen is still small and I prefer to use headphones for the sound.
If you like it, there is a (not cheap) subscription you need to pay to use it.
I have used it with a Samsung S8 and now a Samsung S9 and it neither stuttered or lost a project.
On an old Android tablet, it will not even move the timeline.
Therefore, it depends on HD or UHD bitrate and Android phone you are using.

I am always in favour of the camera but someone might be on a budget and cannot be able to buy new equipment or Premiere or even install free DaVinci on a laptop he does not have or is old.
I own an Android and an IOS phone, both being capable of recording 4K at 60 fps.
Phones and action cameras like Go Pro or SJCAM as a cheaper alternative, all have small sensors and despite their bitrate need to be down converted to HD because in shadows noise and compression might appear, even outside, during the day.
Also, strong grading cannot be applied (if you want and know how to) because the video can�t stand it, despite 70 Mbps or more bitrate.

So yes, a camera would be better but a mobile phone�s video exported in HD still sells for someone who does not have a camera and is happy uploading videos without any retouching.
It�s just you can�t do a serious work (if that is what you want).
It is like a complementary camera.

I also own an iPad but it has limitations and it is not easy or even possible sometimes to import on the move 4K video from my Sony.
Editing with Android in mobile does not have this limitation and I do not have to recompress or export to another codec like I might have to do with an iPad if it does not �like� the codec. It is not a matter of what is better, it is a matter that Android does this easier.

You can also use your phone at a train or airplane and depending on what you want, upload to YouTube or export stock videos, if this saves you work time.
You just have to know what you want to do with your equipment.