Honestly, I read through most of these comments and you’re not getting a lot of history or helpful answers.
When F1 switched to the new turbo hybrid era, some teams got head starts over others. Mercedes benefited the most from both time (head start) and R&D funds to develop the hybrid engines. Which ultimately lead to their dominance for so many years.
At the time, Ron Dennis was the head of McLaren and played a key role in Honda talks and bringing them back into the sport. Honda wanted to wait a year to do further R&D and research on their own, but Ron Dennis essentially asked (pressured) Honda to enter the sport a year earlier than they wanted to. Ultimately that was a bad call on both sides as Honda wasn’t ready, but they did agree to entering before they were “ready.” On top of this, McLaren designed a “tight” aero car for that year and Honda created their infamous “size zero” engine to fit the tight aero packaging. There was hype for this engine design during testing, but it quickly faded when it became clear that the engine design was uncompetitive and unreliable.
A couple more things in play:
McLaren was very vocal about their frustrations with the Honda PU, but McLaren also had a lacking aero philosophy at the time.
McLaren went through leadership changes throughout the McLaren Honda years both for poor F1 performance but also internal politics.
Overall, throughout the McLaren Honda years there was simply too many finger pointing back and forth between the two parties. If being candid, much of which was coming from McLaren blaming Honda. But there were issues on all sides.
Zak Brown comes in and is kind and smiley as is his personality, but in interviews he would say in so many words that McLaren wanted to be competitive so at the time that meant switching from Honda to Renault as Renault was the safer bet at the time.
Simultaneously, Red Bull was getting frustrated with their Renault relationship and wanted a partner that would ultimately lead to championships…in time.
Red Bull gives Honda essentially a test run with Toro Rosso, and Toro Rosso puts in place several measures to make Honda feel as welcome and valued as possible, like their engineers getting Japanese lessons and sushi making lessons.
Red Bull is impressed by Honda’s progress with Toro Rosso and ultimately switches from Renault to Honda.
Renault was building up their works team, and Red Bull wanted their own engines and not be a Renault customer.