I guess the research side would be helpful as well. I was asking for the actual sites to make the investments through.
I'll take any good resources. I have a sum that is uselessly sitting around and need to invest it.
Ah ok. I will give you my opinions on the brokerages as someone who uses 4 different ones. For the vast majority of people, a single one is fine, but there are some very niche advantages each has that really only matter for advanced use.
First off all most brokerages are free now. Robinhood made a living being the free brokerage, but now they are basically just a discount brokerage in a world where all brokerages have no commission trading. A lot of this comes down to preference, with the caveat that I would avoid robinhood unless your preference is a brokerage that is often down on the busiest trading days of the year, has non-existent customer support, and has all kinds of whacky bugs that are constantly being exploited.
I personally use SoFi, TD Ameritrade, Interactive Brokers, and Fidelity. Of these TDA is my favorite, but it was acquired by Schwab and I really don't know how that will change things in the future. SoFi is great if you're looking for a one stop shop for many finance needs. I use their credit card and redeem 2% rewards in crypto/stocks which has become my main credit card. SoFi (and CashApp) are very much targeting the same market as Robinhood (smaller accounts/beginning investors) but they Square and SoFI actually have good operations teams so if you're looking for a pretty interface and don't care about bells and whistles, these are good options.
TDA/Schwab and Fidelity offer pretty good research tools in their platform, are free, have good customer support, and are well entrenched firms that would take a serious black swan to go anywhere. These are safe choices, but with a big caveat that if you're interested in crypto at all, they don't offer it. Robinhood/SoFi/CashApp all offer both.
Interactive brokers I call the honey badger of brokers. They don't give a single shit what you do. You want to put on an obscenely risky trade on leverage? This is the broker for you. Want to short a foreign listed stock with hardly any volume? They don't fucking care. I love and respect them for it, but obviously not needed for everyone. They have good portfolio custom reporting and margin rates as well, but again, that is if you're dedicating a good bit of time to it.
TLDR; Almost all brokerages offer commission free stock trading. If crypto on the same platform is important to you, avoid the legacy guys. If not, click around on some of the sites and see which UI you like. There are tons of reasons to use one over the other for a professional trader, but for most people there are a lot that get the job done.
As far as resources go, I send this video to everyone who asks me about investing. I do a lot of whacky stuff in my portfolio, but my largest position is always, and probably always will be SPY (low cost S&P500 index)