Hi,
for long time I've had problems understanding this whole dpi/ppi theory and its practical usage. Well, I stil have but kinda less.
For the most part it wasn't so important as I never made anything for print, only for web or for animated sequences in movies but soon it can change.

There is a lot of misinformation on internet, some say dpi and ppi is the same thing and that's probably because differences in software - some apps use dpi, some ppi but this whole stuff refers to ppi and in such case yes, dpi and ppi is the same. In reality there is difference in dpi and ppi.
As I understand this:
- dpi is physical size of the image and some kind of a "language" for printers
- ppi describes pixels density on digital image and its quality but it can also change size of the image(!?)
Let say, we have 3000px x 3000px, 300ppi image and then we resize it to 2000px x 2000px without touching ppi... so, we've changed dpi and pixel density is still 300ppi. Am I getting this right?

Anyway, lately I've stumbled on some conversation between pro concept artist and some random guy. This artist wrote, he always work in 72 dpi(because possible lags in higher dpi) and if he want to print finished piece, he convert to 300dpi... But how? If you do something like that, your computer need to add information that was not there.

I undestand basic things like:
- someone wants 300dpi/ppi image to print on 7 inch x 7 inch sheet. So, 7 x 2,54 = 17,78 x 300 ppi gives 5334 px x 5334 image.

Now, let try do this other way:
1. 300px x 300px image divided by 300ppi results in 1 inch x 1 inch print
2. 300px x 300px image divided by 72ppi results in 4,16 inch x 4,16 inch print
I don't get this thing. Well, I get the math behind this but I always thought higher ppi alow bigger print without quality loss. There is this "rule" that for good quality print you need at least 300dpi/ppi image.
I assume that in above example - (1) allows smaller print with good quality and (2) bigger print with worse quality and it' still weird for me.

Another thing and this time lets go to Krita:
- in Krita, default A4 with 300 ppi is 2480px x 3508px. Now, what happens if we leave width and height and change only ppi to 72. It is still A4?
Also, how big print can I make with A4(300ppi)? A4 and smaller, or A4 and bigger? If A4 and bigger then what's the maximum print size?

Another example:
- let assume I want to make 90cm x 60cm poster. With 300ppi it results in 10629px x 7086px image... which is huge and working with something like this in Krita gonna be at least difficult. How to approach such task?

Next thing:
In Scale image to new size we have:
-Pixel Dimensions
-Print Size
and
- Constrain proportions(I keep this always on)
- Adjust print size separately
What is the best way to prepare print in Krita? What should I tweak? Should I only change Pixel Dimensions or width/height in Print size* or maybe Resolution(ppi)?
How much can I tweak the size of print if I start with A4 and is it possible to start with 72ppi and then convert to 300ppi without messing with quality?

*
I assume both are the same as long as Adjust print size separately is unchecked.