new to Sram hydro question

[eddief]

What is biggish red plastic thing in these photos? And how does it work? One is included with both my front and rear AXS levers, hoses, calipers. And the small silver dome with a hole in the side?

Feel free to point me to a good vid. Thx

https://www.aliexpress.us/item/32568...yAdapt=glo2usa