All you need are HTML templates/themes. If it’s HTML, you can put it into ExpressionEngine. You’ll want to break it out and use layouts and such to keep things DRY and plan out your channels and fields ahead of time. There’s definitely a learning curve, but moving an HTML theme into EE is one of my favorite things to do.
No one is professional, everyone starts from scratch. I am very happy that you chose app designing as your career because mobile app designing is one of the most demanding careers nowadays. By the way, I am also an app designer and recently designed an app for ghost book writers that supports ios and android. For mobile app designing, I use Adobe XD software. This is the best software as most designers use this.
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