Base Apparel using Bootstrap

Hello. This is my second challenge after a long break. I took a bootstrap course recently and wanted to test what I learned so far. In my css, in some elements I used REM and in others I used px. Should I use rem or px when making responsive webpages? Also, I tried to make the page responsive for tablet and laptop too but I don't think I did it right for tablet (I wanted to keep the image's width so I made the div with 6 collumns for tablet). I would also like to know how to make the webpage responsive for landscape mode, I kinda got stuck there. Other than that, if you can offer me other helpful tips regarding responsive webpages, I would appreciate it a lot.