BASIC was never built into DOS. It was a standalone program that happened to be included with the DOS distro, but nothing more.
In older computers (mainly 8 bits ones) the OS shell was the BASIC interpreter. In IBM computers you had that casette BASIC in ROM, so the same thing applied: your OS was the BASIC interpreter. In that age, you had to shift to a proper OS to do proper disk managing (in 8 bits computers you had CP/M built in a ROM, for example in Amstrad and Spectrum computers). Sometimes you had a special command in BASIC to activate the D. OS, or you had commands in BASIC that called to OS functions, or maybe you just had to run it from a disk.
Later on, when MSDOS was adopted as the main OS for IBM computers, a slightly modified version of that casette BASIC was included in MSDOS as a standalone program (GW-BASIC).
If you want to start again learning BASIC, I suggest you to grab your favourite version at
http://www.download-qb.tk . The "best" MSDOS version of MS BASIC has to be VBDOS, but if you want the old-fashioned style you can download GW-BASIC or the compiler BASCOM.
Anyhow, once you have "remembered" the BASIC language, I suggest you to try VB. It's easy and you can even get a life coding in it.
You don't find a BASIC interpreter in the command line 'cause the old QBasic that used to be included in MSDOS 5 and 6 is no longer there. Anyway you can download it and install it.