How to upscale without distortion?

The question is why would you need that? Any upscaling algorythm reduces quality, changes colour count and add to file size.
The lossless upscale can be achieved with upscaling image exactly four times without any interpolation and use of lossless compression to save file. This however doesn't add anything quality-wise, except increase in file size, and serves very limited purpose; most likely you will still distort some colours.
So, upscaling - never.

sabersaikou said:
How to upscale without distortion?

upscaling isn't allowed to be uploaded here so why bother? XD

I don't know where uploader get this image...

Original image https://azurlane.koumakan.jp/wiki/File:JervisMaid.png

I'm aware the in-game resource is only 2048px, but this doesn't look like upscaled to me; at least not from that 2048px one.

The detail on the basket liner/cloth is almost impossible

I think the uploader got the high resolution version by other ways.

Not upscaled definitely, original pngs must be somewhere.

From my test, image size is not 9000x9000 and transparent-png.

I think uploader need to explain where he get this.

He doesn't "need", since we don't mandate that. But it would be helpful.

There are lots of possible sources, artist's social, leaks, other official promo material, etc. Reversing game resource isn't the only way to get these official arts.