Actually, I only want to ask something. I read many raw manga's online and when I see some good quality pictures, I think they use some scanning app in mobile phone. They can use something like that these days, but not all raw manga's are available to be read online or download. In the future, I'm planning to purchase raw manga books, scan it and release it. Only Raw's, not translated. So how did they actually release RAW?

If you are asking about where to purchase them, try Amazon.jp.

What you are reading online is probably a mix of mostly free-view chapters and some volume releases mixed in.

Most serialised manga nowadays are first distributed by the publisher via the internet for free-viewing either by web reader or mobile apps. Each release is by partial or full chapters until a volume is complete, and generally these images are of a much lower quality (50%). Generally these releases are what you see scanned.

When the volume is complete it will be released as a book, and often they are also distributed as eBooks. This is their highest quality form and the also the easiest way to them (unless you can find them via pirate sites). If you care about quality for your scans above all else, you should try buy the eBooks and rip images directly. Scanning the physical book will always give inferior quality in comparison.

Some mobile apps from the publisher will sometimes show the highest quality pages as each chapter releases, but the caveat is generally you have to have a working Japanese/Korean/Chinese phone number and citizen ID.

Older manga series are of course only available via their physical forms, and some others have no eBook release. It is for those series you will have to scan them the traditional way.

To find and purchase eBooks or physical books, or for raws that have already been ripped, simply just internet search the original series title in its original language (in kana). Unless you are trying to scan some really obscure manga, you should be able to find official sources among the very first results. Amazon often does not have the highest possible quality eBooks for viewing, so it is quite useful to Google the manga first and see if better sources are available.

But there is website that provide raw chapters and download it freely, not all series, some of it are incomplete.

But there is website that provide raw chapters and download it freely, not all series, some of it are incomplete.

That is an unofficial site, a pirate site.

Similar to how our scanlation aggregates like Mangadex and Batoto work, users ripped the raws and they were uploaded (illegally). There is much less demand for these than English-scanlation work, but they are also very sensitive to JP publishers.
In recent times there was a big crackdown on these sites, Japan introduced law to make them strictly illegal and quite a few were taken out. Of course, this didn't stop them at all, but now it has to be profitable for the site owner.

It is up to users to rip them in the first place, so they generally only list popular manga... and naturally they don't update all series reliably.

I guess quite a few scanlators use these raws sites for their translations, you could too. If you just want to scanlate (period) and don't have any specific manga in mind, it's a good starting option. And free. Whatever you scan will be pretty prone to competition though.