In the office those same people would probably sit on YouTube and switch tabs when their boss walks in. People always found ways to look busy - now they just don't have to. This isn't a problem with working from home, it's a problem with work culture in general.
That said, some people are definitely more capable of working from home than others. I had colleagues I found out were in a different province entirely, at the beach, and nobody had even noticed. They got their work done so who cares? On the other hand I know that some colleagues play games half the day.
If you see me at the mall in the middle of the day, it doesn't mean that I'm slacking off work. It means I took the time to go shopping outside of the typical after-work rush hour, using my lunch hour to do something productive. If it takes longer than my lunch hour I just work back the time.
Rather than pretending to work, I go do something, then come back and get real work done. It's a huge productivity boost.
If staff are replying days later on Teams, you should probably follow up if it's urgent. Teams doesn't do notifications well, so it's easy to miss a message. It's also very possible that your company hasn't hired enough staff, and they're drowning in work.