Problems creating a photo album: which is the expected size and resolution?

Question

First of all, sorry for the basic question: I don't know much about photography and I want to create a photo album. I have already created some in the past using the software provided by the company where I used to print them. However, the quality has dropped so much recently that I want to change the company. Then, if I ever want to reprint any of my albums, I depend on the software I used. Therefore, I decided to create the album using GIMP and then load the final images to the software of the new company (can I say the name of the company? It starts with "Saal"...).
My album will be 21x28cm. I think that the desired resolution to print is 300ppi.
Problems:

When I create a canvas of 42x28cm (two opened pages) with 300ppi in GIMP I get the popup "You are trying to create an image with a size of 152.9 MB". Is it normal that the image is so big? I understand GIMP does not compress to work on it and at the end, to save the final image, probably it will be compressed, but still I find it quite big.
I want to start loading one HEIC image. 1.95Mb (huge size difference!!! I know it's compressed, but...). 4032x3024px. Based on the 300ppi resolution, this image should be ok for a 13.44x10.8in page, which corresponds to 34.1x25.6cm. However, when I save this image with GIMP and I export it (with minimal compression, 30Mb) and I open it with the software of the other (Saal) company, the image appears not covering the whole page, but it appears much smaller than I expected. I know digital images don't have the "size in cms" property, but I thought if I already prepared the image with the exact amount of pixels to cover the whole page at 300ppi, the image would appear quite big, not so small. Well, that might depend on the software of this particular company...?
Then I resize the GIMP image to cover the whole page in the other software. I see the program tells me the quality is good, even when I zoom it much much much more than expected. If I load the HEIC image directly, it says the image is ok, but when I zoom it a bit, it complains already about the quality. So: using GIMP I am telling the image has more quality than it actually has (probably because I created a huge canvas).

All in all, as I said, I'm not a professional, and this is confusing me quite a lot. Am I doing it right? To sum up: if I want to create an album, is it a good idea to use GIMP and then load the pages? Which canvas configuration should I use? Is there a better way to go?
Thank you!

the_limey · Answer

Yes, you are 99% correct, and yes it is confusing.
There's a "DPI" (Dots Per Inch) parameter in image files. Saal uses this when you first import an image to size it, which is why it seems "small". But all that matters is the number of pixels in the image. As you say, 4032x3024 is big enough at 300dpi for 13x10 and that's all that matters when it comes to making your photo album. The small initial size isn't indicative of a quality problem.
So you can just ignore that it initially shows up smaller in Saal, and just make it bigger. Saal will tell you if an image will print with good quality (as you've already seen).
If I was to simplify your workflow, I'd do it like this:

Open the HEIC file in GIMP. Export it to JPEG 100% quality at full size (no downsizing).
Open the JPEG in Saal and enlarge it as you see fit, so long as it's still rated good quality.

Notes:

Many applications can't deal with HEIC files very well. Which is why I suggest using GIMP to convert it to JPEG vs. Saal apparently doing a bad conversion.
The typical printer does 150 lines-per-inch for a photo book, so 300DPI is more than adequate.

Problems creating a photo album: which is the expected size and resolution?

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