Welcome!
A Roku player is just that: a piece of hardware that connects to your TV via HDMI and requires a network connection to the Internet for content. Other than buying the Roku, there is no cost to use a Roku. There are many, many channels (apps) available that are free of charge, but those free ones will generally have advertisements. Depending on the channel, those ads can get really annoying. Roku themselves offer one free channel, simply called The Roku Channel. It has a wide variety of content, and the ads are not overbearing (typically).
As for paid content, there's lots of that as well. Some is fairly inexpensive, and the lower cost ones might still have some ads, just not as bad as the free channels. For example, Peacock (NBC) offers a tier with limited ads, and a higher priced tier with no ads. Netflix historically has never had ads, but I've heard they now (or soon) will have a lower cost limited ad tier. YouTube is free, but has a lot of ads.
If you subscribe to Amazon Prime for the free shipping, your membership also offers a lot of free content. But you have to be careful when you select something, as some titles do require either a rental charge or a purchase (which costs more but offers unlimited streaming).
If you have bought DVD or Blu Ray discs over the past years, they might have included what is called a Digital Copy, meaning you receive a registration number and you can watch what you purchased on a number of different providers.
Note that there are two ways to subscribe to a service such as Netflix or Disney+. One is through Roku, which is a convenient way to set up and maintain your subscriptions. However, be aware that if you subscribe through Roku, you can only watch your subscriptions on a Roku device that is registered to your user account. You won't be able to access the same content on a different device, including a web browser. The second way is to subscribe directly with the provider. The advantage there is that you can watch the content on any device that supports that provider, including a web browser.
Your cellular phone plan might offer access to a provider. For example, my AT&T phone plan gives me access to MAX, which is the new name for HBO Max. Other providers might offer the same, or some other service.
How to connect your Roku to the Internet? It sounds like you have two different options. Your cell phone likely has what is called a Hotspot, and you can use your phone's data plan to connect your Roku (via WiFi) to the Internet. However, if you don't have unlimited data on your phone, you could end up paying a lot of money for exceeding your data cap.
You also mentioned you have WiFi where you live. Assuming that WiFi offers a connection to the Internet, then that's all you need. You don't need an extremely fast Internet connection for successful streaming, Even streaming a 4K movie you don't need more than about 20 Mbps. 1080 Media can stream at under 10 Mbps generally without a problem.
Dan
Roku Community Streaming Expert
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