Krita is developed for painting. Many raster image editing tasks go well, too. You expect vector graphics. To draw it you must insert a vector layer. There you can draw and edit Bezier curves and preset shapes like circles which are freely scalable like in Inkscape or Illustrator.
I guess you would like to see painting tools working as vectors. Unfortunately most of them work only in painting layers (=bitmap).
Do not expect advanced functionality. Vector graphics toolset is rudimentary But at least vectors exist and you can copy and paste stuff between Inkscape and Krita because they both use SVG. Read this for more details: https://docs.krita.org/en/user_manual/vector_graphics.html
In theory painting tools could be vectors which are rendered as accurately as needed to bitmaps for the screen or printing. Nobody has implemented such complex vector objects. As Bezier curves or other elementary vector shapes a rich brush stroke would generate thousands or tens of thousands small paths. It can be made to happen to some degree in Illustrator or Inkscape, but the number of the shapes become soon intolerable.
An alternative way to scale bitmap paintings to bigger size without making them pixelated nor unsharp is to use an image enlargening program. Smilla is a quite well working piece of freeware and there are many commercial ones, for example On1 Resize. They guess often succesfully what should be kept as thin and sharp and what is going to be a smooth gradient. Try them.
BTW. Do not paint for Instagram. Use as high resolution as your computer system and storage possibilities allow. Publish downscaled low resolution versions in web.