Who should own the technical decisions Line manager or Project manager? and Why? Can you provide an example just for my understanding?

For the most part, project managers today possess an understanding of technology rather than a command of technology, yet persist in trying to make technical decisions on the project. This usually infuriates line managers to the show him who’s boss. The size and complexity of today’s projects should make it clear to project managers that they must rely heavily upon the assigned subject matter experts and functional leads for technical direction and support. On some projects, such as in R & D, project manager assignments may be dictated by a requirement for a command of technology rather than just an understanding, but this is an exception rather than the rule. Good project managers know their limitations and never try to dictate a solution without first consulting with the true experts.

I fully agree with the answers been added by EXPERTS......Thanks

Usually there are no conflicts with the person in charge and #1 responsible "Project Manager".

The Project manager does not know everything and should listen to the senior specialists such as a MEP Senior Engineer in HVAC or the consultant or seeking a third opinion rather taking a chance if it's not his area of expertise.