this stuff was confusing to me for a while when I first got into FSX -
mesh = terrain, the shape of the land
landclass = scenery demarcation, the stuff on top of the land like, roads, shorelines, etc
scenery = buildings, trees, airport configuration etc...
landclass and scenery are tied together because landclass will tell the sim where to place the scenery...in the case of trees for instance, the landclass determines where forests will be and from the scenery library the necessary trees are taken and placed there...
then you have 'environmental' addons like REX (payware) and FSWater configurator (which is free)
that alter the sky, clouds, lighting, weather, etc...or the appearance of the water
FSX has by default some decent aspects - but for the most part ACES simply provided placeholders for this tremendous 3rd party development market - some free and some payware
I have bought things that I no longer use - and I have freeware that I would not have minded paying for its that good...so
read the forums and when you see screenshots that you like - ask the poster what they use...you can save a lot of coin
I use REX and FSWater config. for environments and GEX and FTX freeware Pacific N'West and Tasmania along with some beautiful scenery and mesh from Holger Sandman that is free -
you can use google to see which are free and which are payware.