Anyway, am I on the right track? Is this how you enter the market?
First of all, NFT is a really competitive market to be in right now.
The key is promotion. You can either make a really beautiful, stunning, one-of-a-kind digital art or make a crappy or ai generated digital art; either way, you need to promote it.
To be profitable with your NFTs, you need to grow your community as well since most NFT buyers check the social media channels and Discord group first before buying. If there isn't much interest in your NFT then it would be hard to sell it.
How do I know if the possibility of getting something I created is really in circulation?
If you minted an entire collection, you'll start seeing an increase in NFT owners if your own NFT collection gets enough buyers for it to circulate.
And given how easy it is to get my work out there, is it possible for people to take other people's art and sell it instead of the authors?
Yes, someone can steal your NFT and sell it as their own. Although some popular NFT marketplace like Opensea tends to ban or delete those fake collections
IF they noticed it.
I tried the Polygon network because there is no fee and I have no idea if my content is actually valid.
Looks valid to me. You can check these details on each item's activity. You can see in the list when it was minted, put on sale, and also bought.
Here's an example:
https://opensea.io/assets/matic/0x2953399124f0cbb46d2cbacd8a89cf0599974963/84761157435633005768254131177939094636462866350729362710376150310854013747201Last note; the choice of blockchain where you mint your NFT is really up to you. If you choose Polygon that is fine as well, with cheaper gas fees. If you've got enough budget to mint on ETH then go for it since most of the top-performing NFTs are circulating on Ethereum.