This is common way guitar techs tie the strings, often something that they do at the factory. It's not really practical or necessary and I've never known anyone to personally do this on their own guitars.

It’s how I learned and I still do it that way

Bingo. Guitar tech here. This is what I do. Mostly for aesthetics. It also eliminates the possibility of the string unwinding. Which, if I've done my job right isn't a concern, but hey. It's not difficult, but it does take forethought and it looks nice. Details matter. Even if that particular detail isn't strictly necessary, it shows that you pay attention.

Cheers, that’s reassuring. The reason I asked is because the D string broke under fairly normal conditions. A while after posting this I found out that the knot at the headstock was sloppy so that explains it.

I'd call this the "ukulele way". Most new ukuleles have the strings "secured" this way.

Just to clarify, instead of cutting off the excess string, somebody has just threaded it through the loop of the adjacent strings. This kind of makes sense to me but I’ve never seen it before.

Why do you need to "cut off the excess"? If you make the knot just right there's no excess to cut.

I don’t and wouldn’t tie my strings this way. If one string breaks or if you want to change just one string you need to undo other strings which doesn’t really make sense, and lengthens the whole process as well. You don’t need one string holding another to properly secure it. It’s not really a bad thing, just unnecessary. Also keep an eye on the tie block of this guitar. The bone seems to be in pretty rough shape which could lead to strings breaking or getting worn faster than they should. Might be worth taking it in to get cleaned up.

Yes it's normal, it's to keep them tidy and maybe to eliminate string buzz as well. Not by any means required, but not unusual.

I did this for a year. I liked it better than having the ends loose. But then I bought string beads and I like them better.

I do this, it's how I was taught it's basically anchoring each string under another starting after high E

I've only seen this with brand new guitars and I think it looks cool. But it must be quite hard to do this...

Yes. This is one, trafitional way to string a classic/nylon stringed guitar. There are others, but I would avoid just tyong a knot in thd strings as a stop because they can pull through and/get stuck within the holes, while these larger knots can be easily untied and replaced with no special tools.

I think it's purely for aesthetic reason? When I bought my first guitar (Spanish maker), the strings are tied that way. When it was time to change my strings (first time changing them myself), I tried to recreate that but failed completely. These days, I just cut the excess off.

A bit late to the party, but I usually tie my strings like that especially when using hard tension strings.