How to handle/process large text of text file?

I know there are already similar questions ask (like:“how to show large text.”),But this is different. I have Large-text-file.txt(500,000 to 800,000 characters). I don’t want to display that text directly to the user in app. It is like downloaded txt-file in AppSpecificDirectory from web. I want to get usefull data out from that File.
if I use variable to store text before using “split-at”(at 'specific-keyword") then app crashes in some devices.
you can take example of wikipedia-text or page source as text-file.(just example, actual text is different)

→ how to get that text from txt file then split-at words ?.
→ How to read from specific (like read/get text from 400,001 character to 420,000). if possible then how to get-content-size.?

Thanks in advance

Already asked in Kodular community

PS thread closed