Is it okay to trust battery info from a random site I found while stuck at home?

[elegantdha@ege.uniforum.app]

I wanted to ask something a bit different this time, because it is not really about my car but about how I decide what to believe. Yesterday my Suzuki Swift refused to start after sitting in the driveway all weekend, and I spent almost two hours on my phone trying to understand what was going on instead of just calling someone. I ended up reading a lot of pages, and one of them was this website, which explained battery sizes and what the numbers on the label mean in a way that finally made sense to me. I did not buy anything from it, I just read, but it made me realize how little I knew before. My older brother thinks I should only trust a mechanic standing in front of me, and he says anything on the internet is half selling and half guessing. I get his point, but a mechanic can also say whatever he wants and I would not know the difference. So what do you all do? Do you read first, then compare with what the shop tells you, or do you go to the garage and listen only to them? I also wonder if there are simple things I can check myself before anyone gives me a price, like whether the terminals look dirty, whether there is white powder around them, or whether the battery looks swollen on the sides. A neighbor told me a swollen battery is a bad sign, but another person said it is normal in hot weather. Which one is right? And is it worth buying a small voltage tester, or is that something only mechanics need? My car is about nine years old and I honestly cannot remember ever changing the battery, which now sounds a bit scary to me. If someone has been in the same position, with a car that suddenly will not start and no idea who to call, how did you handle it? Did you jump it first, or just wait for help? I would love to hear your simple steps, because next time I want to feel calm instead of panicked. Thank you for reading, and sorry for the long message.