i see these everywhere and i just wanna know how

You can literally take any image and add things like your username, avatar, things that interest you, a quote, etc. as well as boxes for things like "listening to" or "mood" or something.

My announcement template is literally just a blank Ultramarines Chapter Banner from Warhammer. Every time I use it, I have to type everything out, but I always put the same things (in order from top to bottom, my username, my mood, what I'm listening to, the announcement itself, and a quote from Roboute Guilliman, the Chapter's Primarch)

Same way you'd make a normal template. Do you have an image already that you want to use?