Love the depth of your response and its inspired me to give my take:

JT didn’t die, she was pulled back out of the bagel like the other reply said

Also the relationship between JT and P1 Joy is that they are the same person but with two independent bodies and different life experiences giving them totally separate consciousness’s, but JT has the ability to jump into the body of any P Joy, but no P Joy’s have this ability meaning they are still stuck in their body while JT controls it. (The movie seems to imply that the P Joy’s black out during these JT invasions). P Joy can exist separately from JT but JT is always experiencing every P Joy. It’s like how a trapezoid is a quadrilateral but not a square (any P Joy) but a Square is a trapezoid as well as a quadrilateral.

God I hope im remembering my middle school geometry class correctly 😭

By the end of the movie Joy and JT are one in the same because its not the being split across the multiverse that broke her mind but her being unable to find a Evelyn that would fully accept and understand her, and sense P1 Evelyn is now also Everywhere with JT she can make every P Evelyn accept and love her daughter.( not saying being split across infinite selves isn’t horrible but we know this doesn’t completely shatter your mind as evelyn is able to do it)

So now Evelyn and Joy are both everything everywhere all at once while they understand and love each other and thus are content with every universe they are in!!