I work at one of the mega-cap tech companies and manage a large team of engineers. It's extremely clear to me based on various pieces of information that I have access to that we'll be having a significant round of layoffs sometimes in the next quarter or two, yet I have zero involvement in the process. I suspect that at some point I'll be officially "told" that it's happening; perhaps the morning of?
But in every case, I was either consulted for input, or at least given a courtesy heads before the actual layoffs occurred.
In one case I was looped in a few weeks out and asked to help narrow things down. In another case I was simply shown a list of names the night before and offered a token opportunity to object. The list was sound, and I'm pretty sure my boss was doing me a solid on that one by sparing me the hardest decisions.
Line managers aren't always looped in for a few good reasons. Mainly, to prevent leaks. But also I think it can go a long way towards maintaining morale in a post layoff environment. There are countless anecdotes in these threads with people claiming their managers had no idea. Those same employees are now far less likely to be resentful of their managers for laying off their friends.
Best of luck. I don't envy being in your position.