What is the diference between armor and FLAT armor

I have never heard "flat armor". There is "flat armor penetration".

You armor value is made by base armor, which is the value that your champ has from level 1 and increases with levels (usually something between 20-80), and bonus armor, which is the armor you get from items, masteries, runes, abilities.

Armor penetration is the amount of armor that is subtracted from target's armor when calculating damage. FLAT armor penetration, which is linked to Lethality, is the flat amount of armor you ignore from your target. %Bonus armor penetration is the percentage of Bonus Armor you ignore when damaging your target.

There's also %Armor Penetration (in Masteries) which applies to the whole armor, or %Armor Reduction (which implies that everyone ignores the armor of your target, not just you).

U build armor. Champions have a flat armor stat. Their basic armor stat.

Same thing.

Flat just simply means a FIXED amount of armor. A FLAT armor stat will never change.

Regular armor is armor that is prone to change

if you're talking about % vs flat armor pen % deals damage as if you had that% less armor and flat armor deals as if you had the flat number less armor

there's only one armor type, only multiple ways around it.

Its either flat armor or scaling armor. For example Kennens E gives him 50 flat armor and Rammus W gives him 70% of his armor. Kennen will always have same value no matter what items he get while Rammus will get more benefit as he builds items. Flat armor is good early but falls late while scaling armor is not very good early but is great laye