Why most of the apps developed are shitty?

I am an Android developer with several years of experience in the industry. Unfortunately, I've noticed a concerning trend in the quality of software, particularly among apps from so-called unicorn startups and top multinational corporations. Many of these apps are riddled with bugs that seem to slip through the QA process. Here are some examples of reproducible issues I've encountered:

1. In Paytm, when navigating to train bookings and accessing the PNR section, attempting to copy the PNR may result in the screen freezing upon scrolling up, with a blank white screen appearing when scrolling down.
2. Within the train module on Paytm, if the application process is terminated, an infinite loader may appear, and returning to the previous screen may lead to a white screen.
3. PhonePe recently addressed a blank screen issue on the payments page when the process was terminated.
4. PhonePe also fixed an issue where an infinite loader appeared when checking UPI balance, especially after the process was terminated on the UPI pin screen and the back button was pressed.
5. YouTube experiences sporadic crashes when the application process is terminated while using YT Shorts.
6. Instagram had countless bug in the past.
7. Reddit backstack management and deeplinks are hell.
8. Hostar has no security. I was able to decompile and make a debug build with code changes with my novice reverse engineering skills. The room database is even not encrypted.

It does seem like some companies are conducting testing just for the sake of it.

How are the codebases structured for your company? Is it common for companies to produce spaghetti code? Do all companies rely on manual testers.

TLDR; what does the codebase look like in your company? Are the tests adequately written, and to what extent?