It gives everything as much RAM as it can incase they need it, so say application A needs 500mb, but the system has 4GB free, it will give it 1.5GB just to be safe. That way, if the app suddenly needs more RAM, it can access it without the user noticing any delay. As you open more applications, this 'free' pool of RAM obviously decreases, but as it's giving things a very generous overhead it's rarely a problem. It just reduces this overhead for each app based on your use, if it's sat in the background for several days, it's getting the minimum - whereas something like Safari which you're switching to all the time, will get a priority.
Just open your activity monitor and look at the memory pressure, ignore completely the 'used RAM' sections as they mean nothing. Also, find an app that you haven't opened for a while but is still in the background, then look at the RAM usage - open the app back up and you'll see it suddenly increase; this is just the system working.
For what it's worth, I've currently got Affinity Design, Illustrator, Photoshop, InDesign, Cinema4D, AutoCAD, SketchUp, Safari (30+), Chrome (30+), PowerPoint, Excel, Word, Music, Acrobat, Mail, WeChat, and a VPN open. I'm 'using' 12.45GB out of 16GB RAM (iMac), with very low green pressure (Weirdly PowerPoint is using the most RAM...). If you're using these sort of apps on a MacBook Air then I think you've brought the wrong machine as RAM will not be the problem!
Basically, just don't worry about it. As another user pointed out, normally you know if you need more RAM, and the base model configurations are designed to accommodate for the vast majority of people, with upgrades being available for those who need it. But the cost of the upgrades is pretty high, and it's a shame someone who uses it to write the odd email manages to convince themselves they should pay this high price for RAM over say internal storage - which for most people is a better proposition.
Anyway carry on!
