1.5 mm2 is roughly 16 AWG and commonly used for circuits up to 13 or 16 amps in Europe. I have no clue why they're using this type of splice but there are so many Youtube videos on this technique that I've concluded it must be popular. The only reason I can think of is that they want to bury these splices in an inaccessible location (one of the videos shows them plasterboarding over a junction box) and most European regulations stipulate that only maintenance-free connections (soldered, welded or crimped) may be used in inaccessible locations. The wording varies a bit, in the UK it's "maintenance-free" while in Germany it's "non-undoable" or "permanent", which rules out Wago connectors. I suppose that wording goes back to when the only real alternatives were either screw connectors or permanent splices (soldered or crimped).
European device boxes tend to leave little room for conductors and splices so junction boxes are somewhat common but customers often dislike visible covers on walls, so I guess that's one alternative. Another would be using deeper device boxes, pretty popular in Germany these days.
Welding is definitely better than twisting and taping, also shown in quite a few Youtube videos from Eastern Europe! BTW, I don't understand any Russian but I think the chap in the video I linked to talks about Wago connectors and probably gives reasons why he prefers welding.
