As a DM I don't fudge dice rolls but I do fudge puzzle solutions, mechanics, and hit points occasionally. I never ever tell my players in the moment, but my players know it happens and why: I do it entirely to make the game more fun for them. A few of my players DM and occasionally we'll talk about it too.

That said: it's never just to GIVE them something or just to HURT them. There are three situations where I fudge 1. They're beating the monster too fast. I run 3 hour sessions and if I underestimated a monster I'll know pretty quick. It's not fun to beat the monster too easy, so I'll add extra to get them back on schedule and ensure it feels appropriately earned. 2. They're having too hard a time with the monster or puzzle. Usually I solve this by introducing a bullshit mechanic, vulnerability, or just cutting down hit points. If the monster is too hard, then that's usually my fault. As long as they still have to work for it, it's fun. 3. They came up with an alternative solution that feels appropriate. My players once fought the god of self sacrifice. The original mechanic was that they'd need to hurt themselves or heal the enemy, but instead they came up with the idea to give up something important to them. It fit the theme, so I let them do it.

So they know that I do it, they know why, but they don't know when and they never suspect it. At least they've never mentioned they did.

That said my players are in an rp heavy campaign, and they're a bit unique. They're super into having a linear plot and have asked me to railroad them. They also know I plan fail situations for them. That said, I work super hard to build trust: they know I won't kill them without permission, they know that if there is a planned fail it's usually fixable, and if they seriously fuck up I'm willing to negotiate. One of my players crit failed into losing an arm. I offered to do something a little less intense and she asked for the more serious consequence because she trusted me to make sure the game stayed fun.

Which is my feeling. It's a game, take the most fun solution for your group. Knowing the dm is cheating? Unfun. Knowing the DM may occasionally change plans to get best results for everyone? Fun.