Those are two separate issues. Almost all features of Instagram, except for interactive widgets in stories, are available from the web (doesn’t matter mobile or desktop). Like with Facebook, I chose to not install the actual app on my new phone, but I do fire up their web versions sometimes. Considering the extent of functionality, Instagram actually works really smoothly, and I hear the same about FB’s non-basic web version. Though yes, they do require an account.
Speaking of Twitter, it also works well from the web (albeit with no support for their equivalent of stories), but curiously they block VPNs (or perhaps just EC2 IP ranges) in a way that completely breaks some of the site (such as user profiles).
On Twitter/TikTok I can watch any link without having to login. Twitter even allows anonymous search.
Not exactly. When viewed in desktop browser, photo upload is not avaiable and you need to use developer tools to simulate mobile web browser view which is a workaround. Also, even in mobile web, you cannot upload more than one photo, even though this functionality exists in the app. These are very fundamental features of instagram, I think these are not so complex to implement in web versions.