How can I convert the garage to be a loft or studio apartment?

I would set up the largest part of the L as the area you will use the most. Say as your living room area with a comfy sofa and chair with a TV console. Save the smaller section for your bed and dressers, where you'll spend the least time awake. Then add some throw rugs and some artwork on the wall to give it a homey look.

First thing to do is put an exhaust fan in the bathroom or you could have a problem with excessive moisture. A sola tube would add a nice amount of light to either the bathroom or anywhere else. They make sola tubes now that also incorporate exhaust fans into them.

You will want to define spaces. A space for your bed, space for living room stuff, kitchen, etc. Book cases make good dividing items and have a function also. There are tons of room dividers to choose from and the good thing about those they are very easy to move.

I would consider a bed frame that is elevated more then normal. I bought one a few months ago that has 14 inches of storage space underneath it. No box spring the mattress sits right on the bed frame. I can't really explain in detail go to amazon.com and type in elevated bed frame into the search bar.

You will want furniture that will do double duty. An ottoman that opens up for storage. I have seen couches that also lift up for storage in what is normally wasted space.

Light sheer panels would make a nice soft canopy around the bed and define the area. If you want even more privacy go with a thicker material.

Yes an exhaust would be essential, you could put the ceiling type bathroom fan type in, or if thats not possible, consider putting a small window in the bath area, and then put a small clip on fan in to exhaust or one of those window double fan with one that blows air in and one that exhausts air out.

You could put in one of those Ikea Kallax open shelving units to define the bed area. You can use those fabric cubes for storage and display cool things... https://www.ikea.com/us/en/catalog/products/10409932/#/20409936

They also have ceiling mounted curtain rods that you can hang a curtain to define off an area. https://www.ikea.com/us/en/catalog/products/70299153/ they also have a corner track

Maybe you could use the large unit that is presently in the corner, as a room divider to define the one large area into separate functions, such as sleeping area, eating, etc.

To facilitate moving large or heavy pieces of furniture around, and to help prevent floor scratches, first get some sliders. (also called gliders)

These come in different sizes and shapes, and are reasonably priced considering how well they work.

There is a site, https://www.freecycle.org/search where folks can post items that they want to give away for free, or free items that they are looking for. I don't know the extent of how much it exists in San Diego, but you might be able to get some items for your apartment this way.

check this site, there are many interesting things for your loft apartment

A loft conversion means that you take an empty space or area that is no longer used for other purposes and transform it into a living space. For example, warehouses are great for this purpose because you have so much area to work with when you convert a warehouse to a loft living space. They are ideal for this kind of conversion because you have a vast open floor plan and high ceilings that allow you to create separate rooms or leave the entire space open if you want to remodel into a sizable loft-style apartment.

Hardly your dollars would be able to buy so much space in another neighborhood. Make sure you do your research and find out how much it would cost to get it done the way you like it. The conversion can be quite pricey, but it will be worth it in the end because you get to live in a unique loft style apartment that was remodeled precisely how you pictured it.