Ok this is a somewhat nebulous question, but let's start with an important clarification:
Dispel Magic only works on magical effects from spells
Jeremy Crawford has clarified this many times but here is one example from the Sage Advice Compendium:
Dispel magic has a particular purpose: to break other spells. It has
no effect on a vampire’s Charm ability or any other magical effect
that isn’t a spell.
See this Q&A for more evidence and exploration of this facet of the discussion.
Dispel Magic requires you to choose a target
Normal spell targeting rules apply here of course (a lot of the confusing aspects of which are covered in a Dragon Talk podcast here). The target of dispel magic can be:
one creature, object, or magical effect within range
Targeting one magical effect
Let us disregard the first two since they are largely covered by your previous question:How *exactly* does Dispel Magic work with multiple effects?.
The third valid target for dispel magic is one magical effect within range. Unlike the first two, this way of using dispel magic will only affect one magical effect.
Basically, the magical effect is there to allow you to dispel magic that is not attached to creatures or objects or to target a single effect on a creature or object. Examples of magical effects from spells include: an area of darkness caused by the darkness spell, an illusion created by Minor Illusion, and Wall of Fire (among many others).
The important thing to remember is that you can only end ONE effect because you are targeting one magic effect.
You cannot target a type of magical effect
You cannot declare that you are dispelling invisibility in a 30 ft radius. That will not work because you must choose a target. You must basically be able to point to the effect and say "I want to dispel that". If you pointed to an apparently empty square and tried to dispel invisibility there and there was a creature there, they would no longer be invisible!
This is made explicitly clear in the Sage Advice Compendium:
Q: If dispel magic targets the magical effect from bless cast by a cleric, does it remove the effect on all the targets?
A: Dispel magic ends a spell on one target. It doesn’t end the same spell on other targets.
So you would point to a character and say "I cast dispel magic on that character". And, assuming Bless had been cast on that character, they would no longer be under its effects. However, the other characters that were also targeted by the cleric's bless would still be under its effects.
See this Q&A for a discussion on targeting spell effects that you can't see or perceive. And this Q&A talking specifically about dispelling darkness.
Also worth noting that you must target the target of the spell effect not the caster. Targeting the caster of a spell effect will do nothing to dispel the effects.
So, for the above reasons, both of your examples about removing invisibility and glyph of warding in a 30 ft radius are both not possible. Hopefully, you now understand why.