Full Width Slider and Fixed featured image on posts

Hi,

I am running the Mars & Venus theme from Creative Market, and I’d like to make some changes but I can’t figure this out. I can code it but I sort of need someone to explain how. I’ve tried but it doesn’t seem to work. It complicated when you’re working with another person’s code.

My Issues
I would also like my featured image on each post to be viewed like this https://www.inthefrow.com/2016/03/3-ways-confident.html – when the page scrolls, the image remains fixed – the one at the top of the page.
similar here too https://www.themes-ilgelo.com/mani/demo-food/lecker-supergesund/ (the image with the hand and grapes).

If you scroll down to the middle of the page, you’ll see an image that is full width as well – outside the container, How can I do that. I tried several times using css but it looked very weird. see page here https://www.inthefrow.com/2016/03/3-ways-confident.html

I’d also like to have a full-width slider as well, one that expands outside the container. Like this: https://roseland.sculptureqode.com/full-width-slider/

Help Please.
My site is https://sophiaswardrobe.com

Thank you!