<znip> where does it say channeling mana makes you glow like a lightbulb <znip>
You're looking (hah!) for the Perceiving Magic section on page 280 and 281, specifically this:
Noticing magic is a Simple Perception + Intuition [Mental] Test with a threshold equal to the Skill Rating of the being performing it minus the Force of the magic, or 6 – Force if there’s no skill involved (minimum 1 in either case). <znip> You get a + 2 dice pool modifier on this test if you have any magic-related Active or Knowledge Skill.
Note that it doesn't say you "glow like a lightbulb", but that the spellcasting itself or the effects thereof are noticable. Big difference. Also, consider the following from the first paragraph of that section:
Magic is rarely subtle. Any form of magic (conjuring, spellcasting, enchanting, magical lodges, spirits, etc.) changes the world around it. Sometimes it’s obvious through a magician’s gestures or incantations (magicians seen by non-Awakened people are sometimes called “twitchy fingers”). Spirits sometimes cause the air to shimmer, even from astral space. People have reported feeling chills, dread, or other unnatural sensations they can’t quite put their finger on when magic is in the area.
Only if you initiate into the arts and gain access to certain metamagics and/or work with mana that is safely below your own ability (i.e. Force is 5 or more levels less than your Magic Attribute) is the casting attempt or effects sufficiently subtle for most people to avoid noticing.
<znip> what if your someone like Harlequinn who can probably safely change a force 20 spell <znip>
As the perception test is variable based on the Magic attribute of the spellcaster, it depends. Let's take Harlequin and compare him to a relatively strong magician going by Shadowrun fiction (Magic attribute of 6). As of Street Legends, Harlequin has a Magic Attribute of 30; effectively, he is stronger than your beginning Shadowrunner by a factor of 5, and could easily cast a Force 20 spell without being seen as it would take a Perception test with a threshold of of (30-20=10) for anyone to notice the effects. So yeah, Harlequin is a bit of a special case, obviously

For that matter what about inately magical creatures or physical adepts with permanently active powers are they constantly surrounded by a dull glow as they channel that mana?
Good question; innate powers are usually treated differently than shaping mana, and is not mentioned in the "any form of magic" examples above (conjuring, spellcasting, enchanting, and spirits are specifically called out, but they do cover themselves with an "etc" at the end), and like adept powers it's more of an inward magic than an outward one. Also keep in mind that this is purely when magic is shaped; a sustained spell has an aura in the Astral plane, but unless specifically called out like the Armor spell isn't inherently noticable. Only when the spell is cast do you get a chance to notice the spell, at least as I read it.
This would mean you'd be less likely to be noticed if you were in a brihghtly lit space where it gets drowned out by other light sources but in a darkened alley or similar you'd stand out like a sore thumb. For that matter why do you only glow when casting if your sustaining a spell and chaneling all that mana into it shouldn't you continue to glow as your constantly directing all that force 6 power into X effect?
Again, it's not like a lightbulb, that's only the Armor spell. For the rest, see above from page 280.
Personally I don't recall ever reading anything fluff or mechanics wise to explain that perception check except the perception check itself and there is a lot that as Rosa said that makes it make no sense in the system except as a mechanical check to keep spellcasters from running wild. This isn't like DND where you speak, gesture and have to use material components to power a spell you merely focus and bam magical effect unless your centerriing or using reagents to overpower a spell and that's a different matter.
Again, see above

Shadowrun isn't the opposite of D&D; there are plenty of magicians who choose to learn Centering when they initiate to better handle drain, and one of the key requirements for Centering is that it cannot be subtle. Besides that, even if the magician is entirely immobile when casting a spell there's still a chance of noticing the effects of said casting, as per page 280.