After opening a .jpg and only change image and canvas size I want to save file as .jpg with command+s, but Photoshop automatically want to save as .psd. Looks like it's converted into a .psd after opening, because after choosing save as .jpg the .psd file is also still open in Photoshop window.
How can I avoid these extra steps and save the .jpg directly as .jpg?
This happens when you have added properties to the file that are not supported in the jpeg specification.
An example could be using the crop tool set to "hide pixels" instead of "delete pixels", as well as a bunch of other things.
Everything open in Photoshop is a PSD. There is no conversion, that's just what a file is in Photoshop. You should be aware that ANY editing of a JPEG will cause damage.