Learn Dutch: Het systeem is helemaal draadloos. - The system is completely wireless.

It is built exactly like the English word! Draad means 'wire' or 'thread', and the suffix -loos means '-less' (without). You will see -loos in many other Dutch words you encounter, like werkloos (unemployed) or hopeloos (hopeless).

Could I use volledig instead of helemaal here?

Yes, both mean 'completely' or 'entirely.' You could absolutely say Het systeem is volledig draadloos. Helemaal is just a bit more common in everyday spoken Dutch, whereas volledig can sound slightly more formal.

Why does this sentence use het systeem instead of de systeem?

It is a neuter word. A handy trick to remember is that many nouns of Greek or Latin origin ending in -eem or -um are het words. Other common examples you have probably learned are het probleem and het museum.

Which syllable gets the stress when pronouncing systeem?

The emphasis is on the second syllable: sys-TEEM. This is typical for Dutch words ending in a double vowel plus a consonant, especially when they are loanwords.