used flexbox and media queries to make it responsive

Hello, You use a lot of div where you could use semantic elements instead. For example yo ucould use <main> instead of <div id="outerdiv">. <section> instead of <div id="firstpar">, ...

Your design doesn't work well when it is above 375px but still narrow. For example at 450px the "Why us" section is very narrow and the text hard to read.

An advice to fix this is to avoid arbitrary breakpoint such as 375px. Instead : choose the breakpoint based on the design insteadof a specific spot. To do that widen your mobile layout and determine when it start to become worse than the desktop layout. Pick that point as the transition point.