How does upgrade adjacency work?

So I recently discovered via this reddit post that upgrades should be adjacent to the thing they're upgrading (or other upgrades to it). You can see in the image below that the UI will color the border of adjacent upgrades but not ones sitting by themselves:

The post itself is a little unclear how much that helps, and suggests it gives them some sort of bonus, but has no details and is not 100% reliable. Are we sure it gives a bonus, or does the upgrade maybe just not work in the first place if it's not adjacent?

Recently I'd upgraded my jetpack, and while I didn't have a stopwatch out to time it, I wasn't noticing much difference. Then I looked at the text more carefully:

Is this implying that the upgrade won't work unless it's actually adjacent to the jetpack? If so, this seems problematic since when the game starts your jetpack already has things surrounding it and can't be dismantled or moved.

Overall I'm left very unclear on if upgrades must be adjacent to work, and if not, what kind of bonus they get when adjacent.

After reading through the comments in the reddit post you included it seems that the upgrades do not need to be adjacent to the base function to work. Try to group upgrades together and group with main feature for added bonuses. I'm trying to figure this all out just like everyone else but this has seemed to work for me.

As Marshall said, upgrades don't actually need to be next to their base upgrade to function. However, there is a substantial bonus for adjacent similar upgrades. I was able to find a guide that has screenshots of the absolute optimal upgrade patterns for full-sized inventories, as well as general strategies for organizing, here.

It's also worth noting this:

Even without the upgrades touching the function they're meant to be upgrading, you still get some synergy, so long as there are two or more upgrades touching.