Here is the basic process using PS that's implemented in camera. This is a tutorial by Serge Ramelii from about 10 years ago demonstrating the process. He demonstrates this technique starting at 4:25 minutes into the video. You don't need 30fps, but you do need a large number of images. In this tutorial, he used about 10 images but I will usually use between 30 and 60 shots. It's not like digital images are expensive. Once you have the images stacked into a smart object, you can delete the individual files. A couple of advantages of this method over using an ND filter is that 1) you can control how much blur you want in post by altering the number of images you use in the stack and 2) you use normally exposed images metered by your camera, so you shouldn't under/over expose your image.

https://www.youtube.com/watch?v=LNzBNLxLPQY

I originally learned about this method from an old article demonstrating how to increase the optical resolution (not number of pixels) of the image using a very similar technique. The method used the concept of dithering, the same concept used to increase the resolution of the old oscilloscopes which used 8-bit circuitry to get 12-bits of resolution. Reading the article and trying it made me realize that movement would blur the area of the image containing movement. It worked.

There are a number of cellphone apps that use this same concept. I've used an app called 'Hydra" for about 10 years to do this very process in-camera. But there are many more, such as 'Slow Shutter Cam' and 'Shutter Stop'. In the iPhone, you can even use live mode to get the same effect. They all use this same basic method.

Here are a couple of images using an app called Hyrda. Hydra used about 50 images to create the final image. Each image was handheld and used normal daylight exposure with a shutter speed of 1/30s.