There are some superficial similarities, sure, but these mainly come down to conventions between various art software and them taking inspiration from each other.
If you instead look at older versions of CSP it’ll be much more apparent that it’s starkly different from Krita’s in terms of looks, as well as general UI language.
That’s not quite right. Paint tool Sai has no relation to CSP. Both are independent program made by different people. Csp as far as i know it’s the new version of the old manga studio and even the resources from manga studio are compatible with the clip studio.
I don’t think CSP uses qt, but i can be wrong. but i know medibang and fire alpaca use it.
To check if CSP uses Qt, one simple thing to do is to look at the binary files after you install the software.
And it seems they don’t use Qt but visual studio instead. Not sure.
A lot of the dll files are fully custom made and the modules might’ve been created from scratch.