My dad has this one dell, he always uses it plugged in and always shuts it down after use, very rarely removing it from the power source and if he does remove, it’d be there for like an hour max before he shuts it down. So far there are like zero issues, runs smooth. Is this a good practice?

It's not the best practice, it would be better to remove the battery, if using it on mains power mainly.

(It tends to deplete the batteries holding power over time.)

The battery is not constantly being charged. My laptop is always plugged in but stays at 95%. I've read it is better to keep the charge at 80% but the built in chargers do not have that option. It's also better if you run the battery down to 20% before charging but li-ion does not develope a memory. Li-ion degrades over time no matter plus they have a limited number of charging cycles. I'm not sure if it matters...

From what I've read, LiIon batteries need to be slightly depleted on a somewhat regular basis, but not completely. From a rather low sample size, I think that does apply. I charge mine until it's full, then unplug it, but not religiously. I tend to leave them connected to the charger overnight, then unplug them in the morning, and recharge when the charge gets to about 50%, but I really don't pay close attention. The number of available charges advertised is full cycles, and recharging from 50% is about half a charge cycle. I think it's better to use the device unplugged at least for awhile, but I'm not an expert, I'm just going on my own anecdotal experience and what I read on the internet. Take that FWIW.

I was told by a fire officer that you shouldn't charge laptops or phones overnight because it's a fire hazard.

Even having electricity in the walls is a fire hazard. We always have to take some risks. It all depends on the risk/reward ratio. I think the reward is enough greater than the risk that I'll take the chance.

It's not specific to overnight, it's about leaving them unattended - if a faulty charger/battery blows up when you're not there, you can't deal with the fire before it grows.

The easiest way to prevent this is to use original chargers/batteries and only order spares from reputable suppliers who are in compliance with the relevant safety standards, (and stop using them if they're damaged).

Also, separating things which might start a fire from things which could be fuel for a fire isn't an unreasonable precaution either.

I've always turned off non-essential devices to save wasting electricity, but PlusNet tell me that their software expects modems to be left running continuously. If it detects the line is dead, it may automatically reduce speed in case there was a problem!

High temperature and high state-of-charge will cause the battery to degrade faster. If the laptop is mainly used on AC power, removing the battery will prolong its life.

No matter what you do, the battery will lose half of its capacity after four years. At that point, the laptop effectively becomes a crummy desktop. This is something to keep in mind when shopping for computers.
Ed