Do all Colt AR15 Rifles with buttstocks have a Receiver Extension with Buffer and Action Spring installed, as I guess that the Colt AR15 Rifles with collapsible stocks do not have them?

Are they easy to install on the Colt AR15 Rifles with collapsible stocks in order to place a fixed buttstock on the rifle?

Thanks.

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Quoted: Do all Colt AR15 Rifles with buttstocks have a Receiver Extension with Buffer and Action Spring installed, as I guess that the Colt AR15 Rifles with collapsible stocks do not have them? Are they easy to install on the Colt AR15 Rifles with collapsible stocks in order to place a fixed buttstock on the rifle? Thanks.

A complete A2 buttstock assembly will have all of the parts.  It is very easy to install.  Go to the AR15 forum and go down to Build it Yourself  then choose the assembling your own lower receiver.  The carbine buttstock has it's own style receiver extension, buffer, and action spring.  The carbine extensions typically have a castelated locking nut and receiver flange plate where the rifle extension is only a tube.

Quoted: Do all Colt AR15 Rifles with buttstocks have a Receiver Extension with Buffer and Action Spring installed, as I guess that the Colt AR15 Rifles with collapsible stocks do not have them? Are they easy to install on the Colt AR15 Rifles with collapsible stocks in order to place a fixed buttstock on the rifle? Thanks.

Well, yes, a receiver extension aka buffer tube, buffer, and spring is an integral part of the AR15 action.

Different types of stocks have different types of parts.