I can get a CETME parts kit cheap

But what exactly is the big differences between it and say a G3 or a PTR?

I see the sights look a bit different. Internal parts don't really look that different.

So what the deal here, whats all cross compatible between each other

The CETME is an early version of a G3 that is basically the same gun minus a few parts being a little different.
The G3 is a full auto military rifle and the HK91 is the semi auto civilian version of the same rifle. They are the same gun except for the couple small parts that are different between full and semi auto. The HK91 and PTR are the same gun but with a different builder and name.

The CETME and HK91 / PTR parts are interchangeable except=
Bolt carrier and bolt head
Cocking tube, cocking handle support
locking lever and locking piece
Recoil buffer
Trunnion
The fire control parts can be swapped but only as a complete set.

PTR is a company out of Aynor, SC making G3/HK91 semi-auto clones on former Portuguese FMP tooling. They go for around a grand, while a real HK will set you back ~$2500. Greek or Portuguese licensed copies will run upwards of $1500. Most G3 parts will be compatible with the PTR. Many genuine HK's do not have the paddle magazine release, while all but the early PTR's come standard with it (the push button mag release is in a very awkward position). I have a PTR-91 and it is an outstanding rifle. Great company to deal with. I have another on my list.

Appreciate it this is exactly what I was curious about

@bigjunk1

So bare with me here

Could one buy a cetme kit for the guts(bolt, carrier, trunion, trigger pack and housing), use a G3/PTR flat to make a receiver, use a HK53K barrel, and in theory it should work correct?

The 53 is chambered for 5.56. The CETME is .308. The PTR flat will work for it but you would need a .308 barrel. A 53k is a very short barrel also, 5.5"? I think. Are you wanting a short 12"  .308 like a HK91k/ G3k? That would be a blaster

My mistake I meant the 51. Yeah the one I looked at was something ridiculous like 5.x"

Basically what I'm asking is as long as I use CETME trunion and bolt/carrier group, the HK 51 barrel should press into the trunion with no issues and theoretically everything should jive together.

I'm looking at doing a HK build to see if I can do one. I don't want to dive in the deep end and ruin an expensive kit is basically where I'm at.

To be real honest I'm trying to get a CETME L kit(extremely difficult to find ), but CETME C kits are cheap and could be fun to toy around with assuming the above works the way I think it will.

The HK51 barrel should work fine.
Putting an HK kit together is not an easy task. I am not saying someone who has not done it before can't pull it off but it's not just Lego pieces like an AR. The welding and the alignment and spacing during the welding are the part that will be difficult. If you can weld you have a good background for it.
An HK51 clone should be really fun.

I think I can do one. My welding isn't top notch, but I make up for that in my grinding skills

Well see. I have a line on a CETME L kit now. If I can build one of those then that will open the HK world for me to start building too.

Don't base your decision on building a Cetme L kit. It's in not comparable to building a Cetme or HK and will likely be a frustrating experience.  If you really want to practice without breaking the bank build a Cetme C kit and save the L for another time.