All the terms are about a section that connects two other sections with the difference being whether the terms specifies where the connection occurs within the overall structure.
"Bridge" is a general term for a section meant act as a connection between two other sections. This doesn't tell use where it happens.
"Pre-chorus" would be a type of bridge, but is specifically a section preceding a chorus. We now have at least a relative position: before a chorus, but a chorus could appear in several places.
"Middle 8" can be described as a bridge, but the notion of "middle" is probably best understood in terms of a 32 bar song form. The form is divided into two main sections
B. Both sections are generated from 8 bars of initial material which is extended with repetition and variation to make each of the two sections 16 bars.
The song Polka Dots and Moonbeams is a good 32 bar song model. The first 8 bar idea is repeated but with alternate endings. The 8 bar idea is played the first time with a half cadence and then the second time for a full cadence. That provides 16 bars total and is labelled section
A. The next 8 bar idea is new material contrasting with the
A section. The contrast is often in both melodic and harmonic material. It will digress from the opening tonality and then return to it. The return to the opening tonality is achieved with a half cadence. After the contrasting 8 bars the 8 bar idea of the opening
A section is repeated for the final end.
The whole structure, using lower case
b for the intial 8 bar ideas and prime mark
' to indicate variation, is this...
a' 8 bars, 1st ending
a'' 8 bars, 2nd ending
b 8 bars, return to a'' ... these 8 bars are the "middle 8"
a'' 8 bars, 2nd ending
The initial 8 bar idea of the contrasting
B section starts at the middle of the whole 32 bar structure and so got the nickname "middle 8." In general use people will probably use it as a generic "bridge" term. If it's being applied with some sense of the original meaning, it will be a bridge, 8 bars, leading to a concluding section repeated from earlier material.
I have been reading a lot of results on Google but none succeeded to provide a satisfactory answer to my question; and a lot of them just outright have conflicting opinions...
Just consider the speaker. Do they show knowledge of the terminology of musical form? The they write or speak in a way that clearly explains the technical jargon they use? If not, don't put much stock in what they say.