Testimonial Slider Mobile-first workflow

Hello! I'm very appreciated you spending time taking a look at my solution and give me feedback on some issues that I am having.

1. I have a slightly issue with the site in desktop view. How can I position the buttons and the background-image of the <div class='testimonial--container'> which is the quote pattern. They are now moving whenever I make the browser bigger. I also took a look at the site with my Samsung A51 (360px x 625px), it looks pretty nice. However the site looks off on my Samsung tablet (especially on portrait view). The buttons shift way almost to the top of the image's container.

2. Is it a good practice to use different units on css codes. I use both rem and px.

I'm open to all kind of feedback so I can improve in the next challenges. Thank you!