I meant joist instead of ratfers

If you're not heating the garage, what is the point of insulating it?

Thanks for the advice. I am in southern California and it’s hot... i am going to search the contractor for the attic foam insulation. Thank you.
By the way, i did some research on reinforcing the ceiling and i could probably add 4x6 lumber for additional support and add drywall. However, two issues came up.
One- there is a notch from the middle of side ceiling to the center of garage door side. Not sure if i can just remove it? Is it functional? Or buffer support?
Two- there are many block bars between each beam on the wall. So drywall won’t stay even. Removing them is also a time consuming job. Is there a easy way to make them even? Or just remove it?
Here are some more pictures i just took. Thank you a lot

Since I can already feel fall in the air around here, I forgot about SoCal sufferers. Consider radiant barriers instead of or in addition to insulation.

I've built a number of homes and as far as I can see none of your garage framing is there for decoration or "buffer support". And don't assume that when you remove some and the garage doesn't fall down immediately, that it's ok.

Thank you for the advice on that.
If removing that notch beam is not a good idea. What is the alternative way to reinforce ceiling? Somehow, i need to close the ceiling after the insulation of the ceiling. If the ceiling grid and tiles hooking up with the few existing bars can hold up the insulation rolls, i would do that.
What’s your thought on uneven wall? Drywall can’t be attached to the lumber evenly... not sure if removing them is the best way.

How far apart are those horizontal 2xs? You could nail new 2x4s flat in the same plane as the corner braces and lay insulation on top. Not sure what's going on with the piece the opener is notched for. It doesn't seem to have a function.

Thank you. It’s 2x4 and 4ft apart. And from door to the wall then i guess 20ft. So i guess you mean to nail 5 Douglas fir lumbers of 2x4 with the joist hanger like this link? I think that would work!

LUS ZMAX Galvanized Face-Mount Joist Hanger for 2x4 Nominal Lumber

https://www.homedepot.com/p/100375190

No, I'm suggesting attaching to the bottom of the long 2x4s at say 24" on center. They would oriented flat like the diagonal braces in the corner. I'd probably use 3" deck screws, 2 per connection. You could drape rolls of batt insulation over that, then install a ceiling if you want. You should scab some 2x4s vertically from the existing long 2x4s up to the rafters to stiffen them up. This will not provide an attic space that will be accessible, so any junction boxes should be located below.

Thanks for the advice. Just want to make sure i understood correctly. You mean when my ruler is a existing 2x4, add a flat 2x4 underneath a existing 2x4. Right?
However even if this solution can help hold up the insulation rolls. I won’t be able to step on that joist to fix or add wire in the future, right? That means i also need to roll out the insulation after each drywall is attached.

Yes, however the insulation can drape over the new 2x4s. You could put up something as a ceiling that's easier to install than drywall. Maybe plywood.

Why is it a better way than just adding 2x4 between existing joists?

A lot faster

Plus you can leave the diagonal braces, which may be required for stiffness and/or seismic.

Can i ask one more question? Joists and wall post is 2x4 but the thickness of insulation is 10 inches
If i put that insulation on the wall, how to put the drywall?? Just push in?

Sorry, R13 for the wall is 3.5 inches fine...