Doubt with apps in redminote 9S


Jan 13, 2021
Greetings colleagues, I open this thread, because for a long time, I have been observing a couple of things that intrigue me.
The first, and for quite some time, is that after installing a ROM in my Redmi Note 9S, from scratch and clean, at the beginning, a compromised file, it let me extract it in the way that the system brings, without any need for application of third parties, even on the external card, but after a while, whenever I try to unzip a file on the mobile, I get the message "impossible to extract", or something similar, may it need some permission? In the end I have always to use the RAR app.

The other question is about the MyDocument document reader, which comes by default. In my case, I would like to open the TXT files, with another program (without having to open the app first and search for the file), because the MyDocument, I get many characters in Chinese.
I have already tried to change the default app even by deleting data from MyDocument but nothing, it won't let me, if I click on a text file or document, it always opens it with the MyDocument, and it's a pain to have to find the program I want, open it, then go to find the document from that app ... etc ...

Thanks in advance