I’m new to cycling. I bought Giant Contend AR 1 road. It seemed to be normal the first time I rode it but the second time the chain felt like it was loose. Excuse my ignorance on bike stuff. I really don’t know what the problem is. Or is there a problem at all? Is this normal? On the video I didn’t change gears.
Judging from the sound and the video, your chain is getting stuck in the rear derailleur pulleys. Sometimes it’s loose and not engaging on the pulley, and sometimes the chain is routed over the metal bit between pulleys so that it rubs against the metal. You can also get this if your chain is way too long, but it looks tight on the bottom. If it’s way too short, you can also get this by trying to force a gear that the derailleur can’t fit, so the chain downshifts or slips off the pulley.
If there was a difference between the first time you rode it and the second time, then there's likely some kind of problem. (i.e. not normal). Viewing the video, I can't detect it from distance, but it should be a low-level issue - The most likely candidate for a new bike is the cables, new ones can bed-in over the first few rides and the indexing goes a little out of line but is easily fixed.
Issues like new cables doing this mean that having the shop do a tune up on a new bike after the purchase is a normal part of the customer experience. A good shop will offer this after a few weeks; if the bike is built well they won't actually have to do a lot, but you feel supported as a customer.
Equally if something more unusual is at play, they can detect it and put it right, a good shop would want to remedy issues like this.
When you just bought the bike and don't do a lot of your own maintenance yet, then it's just good customer service for them to take a look, and it's part of what you paid for. It's also a learning opportunity if you can ask them to show you what's up and if it's a simple thing you can fix yourself next time.