I create digital art in gimp(raster image)I want to embed external SVG graphics(made in Inkscape)into the gimp image.The final gimp file is 300 dpi and should be high quality because of it digital art that goes to print.I am trying to understand about the external SVG that I want to add to the gimp file:
1.Does it have resolution?
2.Can I scale it how much I need without quality loss(the final document(gimp image size) is big-it's 18"X24" 300 dpi) and I need to scale the SVG graphics to fit so Can I scale it how much I need without quality loss?
3.When I close/save the gimp document -should I rasterize the SVG or it can stay vector is it is?(The final destination of the file is print-it's a digital print)
4.If I rasterize the SVG -What resolution it will have?(maybe the documents?)

I would go the other way... link the Gimp elements into Inkscape and save the final version as a PDF. This will keep the vector elements as vectors in the PDF and make edits easy.

AFAIK you can't embed external ressources in gimp (xcf files) but you can store a very limited subset of svg (= the paths) in xcf

1) no svg does not have resolution.
2) so your input image is 5400 * 7200 pixel wide. Yes you can scale you svg to be 540,5400 or 54000 (or 54000000) "pixels". About quality loss it matters about what we are talking about. By definition there will be less pixels in a 54px wide circle than in a 54000px wide. So the larger the wide of the circle, the better its "definition".
3) Does not make sense. Unless we're talking about a path then yes it will remain as vector in the xcf.
4) you can choose whatever resolution you want when rasterizing the svg to image from inkscape.

It is always good practice to keep original format files for any work you do. You never know if you might need it later safe and with all original features preserved.

1.Does it have resolution?

You can consider SVG objects as math formulas (to oversimplify it), they will scale up or down indefinitely without losing quality. But everything we use to display vector drawings is limited, which means bound by size/resolution.

Keep the original Inkscape SVG so you can return and export losslessly to ANY size/resolution.

2.Can I scale it how much I need without quality loss(the final document(gimp image size) is big-it's 18"X24" 300 dpi) and I need to scale the SVG graphics to fit so Can I scale it how much I need without quality loss?

Yes, you can export to any size without quality loss. It's good practice to set your units and document size in the beginning to the relevant ones (the ones you plan to use in GIMP). GIMP will ask you about what size/resolution you want for the SVG before importing it. Once you import it, it get rasterized and you can't affect the quality of that raster any more.

There is option to "Keep paths" during import of SVG, with it you see can see all the paths on the Paths dialogue (usually on Layers dock) and you can manipulate them with Path tool (B) and use them like you would paths in GIMP, so this is a way to keep the Inkscape paths inside GIMP, but I don't expect everything from Inkscape would translate equally into GIMP, and I find Inkscape much easier to use.

3.When I close/save the gimp document -should I rasterize the SVG or it can stay vector is it is?(The final destination of the file is print-it's a digital print)

SVG gets rasterized on import, it will already be rasterized, with the paths also imported so you can manipulate them in GIMP (create new layers from them, you can't influence the imported rasterized SVG by editing the imported paths!).

If you save as GIMP native format .xcf the path information is saved. But .xcf will not be printed, you will export the drawing into a print friendly format, which means it will get rasterized or keep some vectors depending on the format.

File that is generated for printing isn't, in 99.9% of cases, to be re-edited. The only purpose of the file for print is to print it. The process that creates files for print always generates at least 2 files, the original (.xcf or .svg), and the one for print (.pdf or .eps or .tiff).

4.If I rasterize the SVG -What resolution it will have?(maybe the documents?)

GIMP will ask you which size and resolution to use to import the SVG. If you use the 96 dpi which is default in Inkscape, the file will have the same size in GIMP as it did in Inkscape (mind the units!).

If you use the same units and sizes in Inkscape and GIMP (and why wouldn't you set it that way when working on pieces for the same artwork?), if you set the dpi less than 96 you will get lower size (and thus quality), if higher than 96 you'll get larger size/quality.

In your case I would create 18"X24" file in Inkscape, and 18"X24" with 300 dpi in GIMP. When importing .svg into GIMP I would set the size to 18"X24" and import resolution to 300. Or you can export a PNG from Inkscape, export the Page and set dpi to 300 (size will look huge), but this will give you exactly the same quality as importing .svg at 18"X24" and 300 dpi.