I love this conversation about thrifting, especially given how many options we have to do so in Los Angeles. I've only gone a handful of times during my time living in LA and I honestly wish I could do it more!
To add onto the options Jacqueline posted, I've been to Jet Rag once before for their Sunday sale and it really is a fun experience to hunt around for $1 finds. I will say, the quality of their cheaper items does vary, but I did buy a pair of retro sunglasses from their store (albeit not for a dollar) that I still love and wear nearly three years on.
I personally really love Crossroads Trading. This is the consignment store chain I've frequented the most and I do think they have one of the better selections I've seen, especially when it comes to brands Gen Zers love. Although that means they carry a good amount of fast fashion, such as Urban Outfitters, amongst others, it makes me feel better to buy clothing which has been 'pre-loved.' Additionally, these items are usually cheaper than their brand-new counterparts, which makes it more affordable for anyone on a college student budget, and they last just as long!
I also recommend Out of the Closet, another thrift store chain. The nonprofit's sale revenues provide medical care for patients with HIV/AIDS, which means that not only are you benefiting the environment by buying second-hand, you're also helping a great social cause! I've shopped from both their household and clothing sections (pro tip: they have a great selection of books!) and I loved both.
While thrifting is a great way to buy trendy pieces that might break the bank for cheap, it's also a way to experiment with trends that may go out of style without the environmental and social costs of buying something you'll only wear for a few months. I speak from experience; one of my favourite finds is a two-toned denim jacket from a local LA boutique. I found it for a fourth of its original price and I get compliments on it all the time. And while I see myself wearing this even if denim jackets go out of style, it's definitely something I wouldn't have paid full price for and I love it even more because I didn't have to.
I also love thrifting online too. While I haven't dabbled with buying thrifted clothes from platforms like ThredUp as much as I would've liked, my roommate has been quite successful and it's an app I want to peruse since it's definitely better than Amazon. Personally, I love second hand book shopping - ThriftBooks has never let me down and I, alongside my household, have never been disappointed by their quality. While you can't find every title and it's not always as cheap as Amazon is, it's definitely a great resource for buying secondhand literature. My other roommate even bought her textbooks on it!