Can you overcharge the battery on the apple watch

I woke up this morning realizing i left my apple watvh on its charger overnight and i started freaking out because that is something i don't do. but lately my scheduele has been hectic and i tend to fall asleep at random and now i am scared that my apple watch's battery has problems because i just got this two days ago 🏻♀

No. The charger is overcharge protected by shutting off when the watch battery is fully charged. Stop worrying and get a full night's sleep. 🙂

If that's the case then why is my Apple Watch pretty warm when I take if off the charger? I heard on a podcast that other people are experiencing the same (warm watch in the morning). I'm like the OP, I put my watch on the charger when it has 40-50% battery left and leave it charging overnight.

Mine is also warm in the morning. I believe either there is always a small amount of current flowing or the charging stops and starts as the watch charge drops a tiny bit. Either way there is no documentation that any harm can occur but many articles that this does not cause a problem. I do notice that mine charges very quickly and usually by the time I cut the lights off it's at 100% so I could remove it if I were concerned but I never do.

Because, once fully charged, the watch is still using power, so the charger has to keep trickling power to it to maintain the 100%.

The charger will not overcharge - it has circuitry to prevent that

If that is the case, When the battery does loose it's all day life, does apple replace the battery for free?

No. Apple does not replace batteries under those circumstances.

We do that all the time. There's no issue with that. I use the night stand feature and sometimes leave the watch on the charger all day.