dnd 5e – Does Dim Light created by an effect override Bright Light in an area?

Unless the spell, item, ability, etc mentions it specifically changing the level of light in a area/radius, then I would say no, it does not change it. Just think about turning on a dim flashlight outside during the day. It is still emitting light, but it doesn’t make a difference. Now take that same light and go into a tunnel or cave and the dim light matters.

Here is an example of an ability changing the level of light:

The Oathbreaker paladin’s 20th-level Dread Lord feature:

At 20th level, the paladin can, as an action, surround himself or
herself with an aura of gloom that lasts for 1 minute. The aura
reduces any bright light in a 30-foot radius around the paladin to dim
light.

And a spell:

Shadow of Moil:

The shadows turn dim light within 10 feet of you into darkness, and
bright light in the same area to dim light.

To clarify, though: For some spells or effects, such as the Twilight Domain’s Channel Divinity, it is possible to be in the area of dim light from a spell/effect and be affected by it even if you are in an area with bright light… Because magic in D&D does not follow our world’s physics to a T.