So I get the concept of forked coins of bitcoin with clearer view as to from where they come and how they affect the bitcoin reality.
there are 3 things that shouldn't be confused with each other:
1. altcoins forked from bitcoin
an altcoin that is forked from bitcoin is simply an altcoin that copies the code. for example Litecoin is a fork of bitcoin. it copied the code, changed a bunch of stuff (algorithm, supply, time between blocks,...) and created a new coin.
2. the thing we call "forked coins"
these are useless shitcoins that are created from a popular project like bitcoin to abuse the "brand name" and make some money. examples are Bitcoin Cash, Bitcoin Gold, Bitcoin Private,... there is no point in their existence, they are not doing anything and they will die eventually.
these forked coins are both using the code and the blockchain of the original project.
3. bitcoin forks.
you can call them "upgrades". it can be to add or remove functionality, to fix a bug,... for example the soft fork we had last year to add Segregated Witness.
Each halving makes the bitcoins supply fall,
supply does NOT fall. it is constantly increasing with each new block. the only thing that is changing is the "rate" of its increase.