1. In the first hint, it said "decompile", so I search with "decompile apk in kali linux" and found [`apktool`](https://www.kali.org/tools/apktool/) (though Hint 2 suggests mobsf or jadx)
2. According to the [official docs](https://apktool.org/docs/the-basics/intro/), we can decompile the apk with `apktool d file.apk` After running the command, there is going to be a new folder
3.
> You will find the flag after analysing this apk

But I'm too lazy to analyse them one by one, so I search for [the usage of `grep` to look for `pico` repetitively](https://unix.stackexchange.com/questions/21169/how-to-search-for-a-word-in-entire-content-of-a-directory-in-linux)

┌──(kali㉿kali)-[~/code/ti/timer]
└─$ grep -r "pico"
smali_classes3/com/example/timer/BuildConfig.smali:.field public static final VERSION_NAME:Ljava/lang/String; = "picoCTF{t1m3r_r3v3rs3d_succ355fully_17496}"
apktool.yml:  versionName: picoCTF{t1m3r_r3v3rs3d_succ355fully_17496}