There is a thread under fly lines you can search about "welded loops" that many of the same commenters here contributed to. The up-shot was that welded loops as supplied by line manufacturers on most contemporary lines are about 80% the strength of the line (20 to 35 lb. test depending on core material) and can crack. They can be brought up to full+ strength by mechanical reinforcement; I tie a 12+ turn nail knot over the doubled portion of the loop when new and there are other valid approaches. The Bimini Twist is a standard knot among salts that can form a large loop that will fit over a spool of line and is not hard to tie with a little practice; there are many internet illustrations and videos depicting this and I tie one in the backing on every reel/spool I have so I can loop-to-loop fly lines on and off.
This image shows the nail knot reinforced loop and a double line Bimini to reduce smaller diameter line bite into the softer fly line coating.