My players are currently in a futuristic setting where they are being shot at by lasers and such. One of my players tried to justify that an antimagic field should be able to disable future tech because it's "basically magic". My understanding is that all magic comes from The Weave, and anything outside of that is just considered technology (like an electric light bulb). Am I wrong in this assumption?
Dispel Magic Only Ends Spells
A magical effect that is not the result of an ongoing spell cannot be dispelled.
Jeremy Crawford has issued several Twitter rulings to this effect.
In this Tweet:
Q: Can dispel magic undo a druid's wild shape
A: Dispel magic ends spells. Wild Shape isn't a spell.
and this one:
Q: Does Dispel Magic auto work vs Channel Divinity powers? Ie.. Paladin's Vow of Enmity magic effect.
A: Dispel magic ends spells. A Channel Divinity option like the paladin's Sacred Weapon isn't a spell.
Q: Does dispel magic effect an arcane ward?
A: Dispel magic ends spells. Arcane Ward isn't a spell
Q: Does that mean magical items cant be dispelled either? We've assumed that they could be disabled temporarily with dispel.
A: Dispel magic can end spells that come from a magic item, but it has no effect on the item itself.
Also note that Dispel Magic can only end spells with an ongoing duration. It cannot remove the lingering effects of an instantaneous spell. PHB P. 203:
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.
The March 2016 Sage Advice Column covers Dispel Magic as well. It reiterates that Dispel magic cannot end magical effects that are not the result of a spell, such as a vampire's Charm effect. It also reiterates the rule about Dispel Magic not working on the results of a spell with an instantaneous effect. It adds that a readied Dispel Magic cannot be used in place of a Counterspell, though in some limited circumstances, it may be almost as good.
To answer your actual question, in this context, targeting a magical effect clearly means you can target just one specific ongoing spell effect instead of stripping everything on a particular creature.
They are not detectable with detect magic
Detect magic can only detect creatures — even highly magical creatures such as outsiders and elementals — if they are the result of an ongoing spell, such as summon monster. No creature type detects as magical in itself, not even undead. Specifically, the spell has listings for magic items and for spells, and for nothing else.
But they are Magical Beasts
Familiars (emphasis added):
A familiar is an animal chosen by a spellcaster to aid him in his study of magic. It retains the [majority of the game stats] of the normal animal it once was, but is now a magical beast for the purpose of effects that depend on its type.
Generally, any animal that becomes too smart for the confines of the Animal type (2 Int or less) becomes a Magical Beast by virtue of specific wording of whatever effect is responsible for the Int increase; there is, as far as I know, no general rule to ensure this in all cases.