Bought a OEM alternator - quicksilver 807653t Mando 65A: Serpentine Belt

swapping it and I see the old had a external voltage regulator (I think it’s that, the little box mounted next to it and 2 wires to the alternator )

wasn’t sure if I need that or do they now have built in regulators?

More photos. You can see the box with 2 wires. It was hooked to the alternator and not sure if I still need it or if the new ones have it built in.

Most have them built in on the back, but you can get alternators that don't, so save it just in case.

my a. But how can I test or know if I need it or if it’s built in?

Here you go:

you need a good multimeter. Don’t buy a cheap one (don’t hate me, I had one for years too). I bought a nice Klein one, I’m too cheap for fluke (although those are great when supplied by the employer).

a regulator clamps down the voltage from alternator due to it making too much voltage at higher rpm. It needs to make more than 12V but less than approx 14.5-15V at higher rpm. This is especially important for us boat guys since that’s where the operating rpm is most of the time vs cars. this video should show you if you need that regulator or not.

That is not a regulator. Most likely a noise filter of some kind. I would be surprised if you need it unless you are getting a whining sound in your stereo that changes with engine RPM.