How do instagram models downgrade their picture quality?

This picture https://imgur.com/a/h1U7f3o is a good example. It is a professionally looking taken photo by a instagram model but the picture is all grainy like it was taken by a 1990’s camera. Most of her other photos are clear and not grainy like that. I notice this sometimes with models where their pictures are grainy, to me it looks like they are trying to hide imperfections in their skin by making the photo grainy. I am curious how do they do this? Is there a easy way to take a clear photo and make it grainy like that?

Filters.

Which filter makes the picture grainy? Because when I have used filters before all it did was give the picture a slightly different shade of color.

Not filters in the film camera sense, but PhotoShop type filters that combine changes to contrast, saturation, edge sharpness and other settings that can make a photo look different from the input file.

There is a grainy effect on most photo editing apps.

Can you give me the name of a good app?

Adobe’s Photoshop is the industry standard.

For a free option: gimp.

Note that many of these have a fairly steep learning curve. You’ll likely want to find a tutorial on how to use the one you choose to get the effect you want.

Here’s a gimp tutorial

Here’s a big ol’ list of software.

I am mostly using Luminar these days.
It is almost entirely filter-based, and has hundreds of effects.