Can a Warforged have a ranged weapon affixed to them like an armblade or is it strictly just melee weapons only? I was curious if a crossbow could be used in place of it possibly.

Also, is the main function of an armblade just so you can't be disarmed?

Yes, but 5th edition has no rules for doing it

Ranged weapons can definitely be attached to warforged in Eberron; we have plenty of examples of that happening in older editions of D&D. But 5e doesn’t provide any rules for doing so: the armblade component is limited to melee weapons, and no “armbow” component exists in 5e.

Historical precedent: 3.5e armbow

In the 3.5 edition of D&D, the original Eberron Campaign Setting included an armbow attached warforged component right next to the armblade. However, the armbow was vastly more expensive (20,000 gp to the armblade’s 2,300: nearly an entire order of magnitude). On the other hand, the armbow arguably had greater functionality. It added +2 to attack and damage (compared to the armblade’s +1), it auto-magically created and loaded ammunition (which the armblade, obviously, had no need to do), and it could automatically cast align weapon on that ammunition for the unusual price of 3 hp per shot (thanks to 3.5e’s greater power level, recovering this damage was extremely trivial by about 2nd level).

Personally, I’d call the 3.5e armbow preposterously overpriced, even before getting into the implication that it was impossible to fire more than once per round even if you were supposed to have that ability, but that’s neither here nor there.

Anyway, the existence of the 3.5e armbow establishes that attaching ranged weapons to warforged is a totally consistent and appropriate thing in Eberron—but doesn’t answer what the 5th edition rules for doing so would be.

5e situation: Eberron still seems to be “in flux,” and no armbow exists (yet?)

Unfortunately, 5e Eberron is rather difficult to pin down—they keep publishing Unearthed Arcana or test material for it, but Wizards of the Coast has yet to publish anything finalized for the setting, including the warforged race itself. That said, the 2018 Unearthed Arcana article “Magic Items of Eberron” defines the armblade as follows:

Armblade

Weapon (any one-handed melee weapon), common (requires attunement by a warforged)

An armblade is a weapon designed to integrate with the forearm of a warforged. If you’re a warforged, you can attach an armblade by attuning to it. An attached armblade cannot be disarmed or removed from you against your will, but while the weapon is attached you cannot use that hand for other actions. You can spend one minute to end the attunement and remove the armblade.

As the first line there says, it only applies to a “one-handed melee weapon.” So while armbows are things in Eberron, they aren’t (yet?) defined in 5e. Unfortunately, there just doesn’t seem to be any more information available on this subject.

Ultimately, the armblade requires attuning—an extremely valuable resource, since you can only attune 3 items—and then its only benefit is that it can’t be disarmed. “Magic Items of Eberron” even includes a sidebar explaining that armblades are not magical weapons and don’t overcome resistance as if they were—though it does say you can make an armblade from an already-magic weapon. Whether that would now require attuning twice, once as an armblade and once for the magic weapon it actually is, is unclear. If not, that could maybe make an armblade worthwhile, but if so—or if dealing with a non-magic weapon or magic weapon that did not otherwise require attunement—this rendition of armblade is a terrible choice, consuming an extremely-scarce resource for an extremely-small benefit.

Armblades can only be one-handed melee weapons

According to Unearthed Arcana: Magic Items of Eberron:

Armblade

Weapon (any one-handed melee weapon), common (requires attunement by a warforged)

An armblade is a weapon designed to integrate
with the forearm of a warforged. If you’re a
warforged, you can attach an armblade by
attuning to it. An attached armblade cannot be
disarmed or removed from you against your will,
but while the weapon is attached you cannot use
that hand for other actions. You can spend one
minute to end the attunement and remove the
armblade.

But as comments by Rubiksmoose and NautArch to this question and its answers point out: the armblade has also been released in Wayfinder’s Guide to Eberron. I don't own this book, so unfortunately I cannot check how the latest version of this race feature is worded in it. In any case, consult your DM if you are interested in having a ranged weapon as armblade.

Regarding the "main function" of a race feature: as far as I know, this website doesn't cover designer intent.