I get that you have to mill out the trigger area, but how? Do people take them to machine shops? Do you have to register them with BATFE after it is milled?

I haven't seen 95% lowers, but I have seen the 80% lowers. No, you don't take it to a machine shop - you mill it out yourself in order for it to be a homebuilt firearm. People have done it with mills, drill presses and Dremels.

Who's selling 95% lowers?

http://www.americanspiritarms.com/lower-receivers/asa-m4-ar-15-95-percent-lower-receiver/

I guess they are for people with more time and patience than I have. Do you have to register them when you are done? What about a serial number?

Those look like 80% lowers to me.

What is the difference between an 80% and a 95% lower.

Although 15% is a correct answer it is not the one I am looking for.

Looks like an 80% with the rear upper lug pocket started for you and drilled. It would be the same as doing an 80% with a jig. As for the serial numbers no you do not need them nor do you need to register it as far as the BATF is concerned its a paper weight... Please correct me if I'm wrong...

ETA: 95% seems to have the rear upper lug pocket and pin hole drilled, also the buffer retainer hole drilled. All you need to do is finish the fcg pocket and drill the trigger/ hammer pins and the selector hole. All would be on a good quality jig

CNC Guns Milling and Drilling jigs

http://www.cncguns.com/tooling.html

Nothing.

"80%" or "95%" is just an advertising phrase.  The ATF either calls a lump of metal a firearm or it is not a firearm.  There is no inbetween according to law or interpretation. This phrase is worse than "National Match" or "Mil-spec."  

At least Mil-spec refers to Military specifications that are met in some minute way, like the anodizing or type of metal - IF the advertiser is honest.  

80% lowers from TM can be as low as $75 apiece when you buy 5
https://www.tacticalmachining.com/80-products.html

Register with who?

THERE IS NO GUN REGISTRATION IN THE USA (yet).   Some states require it, like Commie-fornia.   Why would you ask about "registration" are YOU from California?
Does Tenessee now require it?  I don't think so.

As for serial numbers: GUNS DO NOT REQUIRE SERIAL NUMBERS.  Many guns manufactured prior to 1968's GCA '68 have NO SERIAL NUMBER AT ALL.  Sears brand .22s are often that way.  

After 1968, guns which were NEWLY MANUFACTURED required serial numbers - but guns which are MADE by an individual do not require any number at all.  It is suggested you make up a pretend, fictitious, not-real, faux, serial number & put it on guns you MAKE, but it is only for your own purposes.

AGAIN: THERE IS NO NATIONAL REGISTRATION

EDIT: once a MANUFACTURED gun has been stamped with a serial number, it is a felony to alter that number.  We are NOT discussing manufacturing here.

I am aware that officially there is no national registration. But why do you have to go through a FFL if no one is keeping track. Don't you have to provide a serial number for a SBR?

GCA '68 (I think, it might have been NFA '34, I wasn't around for either one) is what requires you to go through an FFL.  The firearm can be tracked from the manufacturer to the initial individual that purchased it from the FFL.  After that, it's anyone's guess where it went as most states don't require any paperwork for face to face sales.  In states that have registration, the .gov can look up the serial # of the gun and tell you who owns it.  The federal .gov has to first have the manufacturer check their records and see which distributor it was sent to.  Then have the distributor see which dealer they sent it to.  Then have the dealer see who he sold it to.  Then see if that individual still has it or not.  And 4473s are not registration.  Does the federal .gov know that you have guns because you filled out a 4473?  Probably, even though they aren't supposed to keep records of it.  But no firearm information other than type is actually called in when the NICS check is done.  The dealer does have to keep the 4473 on file though.  4473s could more accurately be described as a registration of you as a firearm owner than a registration of the firearm.  That's if the Feds are actually keeping record of the 4473s called in, which they aren't supposed to.

For clarification, there is a difference between makers and manufacturers.  Manufacturers have FFLs, makers don't.  Makers are not required to mark Title I firearms at all.  Manufacturers are.  The markings required on Title II firearms are the same for both makers and manufacturers.

ETA:  Makers are also not allowed to make firearms for anything but personal use.  This doesn't mean you can't sell a firearm that you made for your own use, it just means you can't make it with the intent of selling it.