Mobile portfolio is not responsive

You can customize the look of every element on your site thanks to CSS. You can add that CSS to your theme stylesheet, or to your custom CSS editor available under Appearance > Edit CSS in your dashboard.

To change CSS only a specific device, you can use CSS media queries:
https://www.w3schools.com/cssref/css3_pr_mediaquery.asp

If you’re not familiar with CSS, I’d recommend getting in touch with the theme author about this. It’s most likely something they’d want to change for all the users of their theme.

I reviewed all CSS file, but still I can not understand why is it. It makes 2 columns “portfolio-entry-first-item-row” and “portfolio-entry-last-item-row”. I think it’s from the function, but I am not good with php.

I’d recommend checking the end of the theme stylesheet, especially the sections appearing inside CSS media queries. There, look for clear entries that force portfolio items to break to another line, or stay aligned to the right of the other items.

You can then add your own rules to change the clear value to what you need.