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Hey, glad to welcome you to self hosting!
There are a few different components to RSS. The feed (in this case Instagram or something like webtoons), an aggregator (the software that pulls in all the feeds you're interested in and keeps track of things like read status), and a client (the actual interface you interact with to read your feeds)
A lot of time, the aggregator will include a web client you can use, so these can be bundled together. But, because RSS is an open specification, you could use a client other than the one that comes with an aggregator. Examples of this are Miniflux or FreshRSS. If you're interested in Nextcloud, there is also an RSS plugin for that
The other part, the feed, is often provided by a website directly. Webtoons does this for instances. For each comic, there is a URL that points to the feed. Some sites will have a little RSS icon that direct you to the feed. While other sites will have you manually add a something likr "/rss" or "/atom.xml" to find that feed
But other sites, like Instagram, don't provide feeds directly. To get those feeds, you'll need some kind of service that scrapes content from Instagram and creates a feed from that. I'm sure there are selfhosted options for this, but because the original content had to come from a third party anyway, I don't mind using a public service to create feeds for me. I personally use https://openrss.org, which doesn't require an account to use, though I'm sure there are others as well. It has support for Instagram and a bunch of other sites too. I will warn that by the nature of having a service that scrapes another, things may break sometimes. I don't follow any Instagram feeds through openrss, but I have some other sites/feeds that I do get through them and am generally happy with it
TLDR: Put something like Miniflux on your server and add the Instagram feeds you want through openrss.org to Miniflux
Does that help?