MTGNexus

with Titania's Song in play and Infinite Reflection attached to whatever, a grizzly bear, what happens when I cast Welding Jar? thank you

When figuring out what replacement effects to apply as a permanent enters the battlefield, we first take into account any static abilities that would apply to that permanent when it's on the battlefield. The classic example of this is Blood Moon, which prevents lands from entering the battlefield with counters.

So in this circumstance, Welding Jar would be entering as a creature, so Infinite Reflection turns it into a Grizzly Bear.

What you have here is a classic dependency rather than the typical "timestamp" situation. Because the groups of cards that Infinite Reflection would change if you applied Titania's Song they are dependant so you apply Titania's Song first.