How legitimate is writing an aggregator site?

Question

Law in IT

Good afternoon, I was interested in the question of the legitimacy of writing an aggregator site. Suppose there is a certain set of sites containing mutually intersecting categories of content, which at the same time lack a convenient interface, filtering tools (for example, there is no corny logical negation "show everything except ..."), logically organized cataloging, and additional tools for adding tags displayed in an algorithmic way .

Since I am a user of all these resources, this state of affairs is gradually becoming oppressive and I am approaching the idea of ​​​​writing a web search engine with the ability to display, comment and "attach" additional meta information to the content to improve search tools. Naturally, this opens up a huge scope for statistical / cluster analysis, building a model of a recommender system, organizing a fast-working search tree, and other fascinating matan. However, I would like to understand the legal restrictions imposed on such systems from the very beginning, especially when the resource provides copyrighted content.

I do not represent any startup or a rabid enthusiast, on the contrary, I understand the enormous amount of work and time it will take and I want to correctly approach the issue from the very beginning.

Answer 1

A little about what has already been said:
1) Excerpt from the question:

...a web engine with the ability to display, comment and "attach" additional meta-information to the content to improve search tools.

Extract from Dr. Bacon :

The eternal problem, Google, Yandex and other search engines, in fact, are also aggregators and not everyone is happy with what they are doing.

CosmoV (the author) talks about a service that actually includes the ability to complete existing content. Neither Google nor Yandex provide such opportunities. Their maximum is a small announcement of an article or a short answer in search results, an announcement of news in a news aggregator. If you want more, go to the site.

...and not everyone is happy with what they do.

Dissatisfied - close the site from indexing. Or play by their rules. Google and Yandex are not web services, but self-sufficient browsers. There's a huge difference here.
2) Excerpt from the comment of CosmoV (author):

Many sites in the user agreement expressly prohibit copying and distributing information without their knowledge, however, the content on them is often written by users who ultimately own the authorship.

That's it. If a user publishes his material on any site, then he necessarily accepts his user agreement (in which there is no word that his content will be used by aggregators like yours). That is, in fact, your actions will be regarded as simple copying. Even with the source. Because the user does not even know about the existence of your site, did not give consent to the publication of his content on your site (except in cases where consent to use is recorded in the user agreement of the site from which you take the material, but more on that below) and did not accept your user agreement.
Possible solution to the problem
There is a Latin expression "Cui Bono?" , which literally translates as"Who benefits from this?" . So, according to the principle of this expression, all interactions of people (and not only) with each other are built. And your situation is no exception. The existence of your web application should be beneficial to the owners of content donor sites. Only in this case they will not interfere with you, but interact with you.
Here you have to think and decide whether your site will be useful for both users and content donor sites. And will it bring something to you (unless, of course, you work on a voluntary basis)? If the answer is “Yes”, then contact the representatives of the above-mentioned sites and negotiate cooperation with them. Let them add the permission clause for publishing content on your aggregator to their user agreement.
But, if things are in such a way that you are confident in the usefulness of your service and that users will appreciate it, then why not create your site with the ability to publish user-generated content on it? And when the site has a reputation, then donor sites will treat it with greater loyalty (it will be easier to agree on cooperation). Although this is a completely different story. In general, it's up to you. Phew, tired of typing...

Answer 2

The eternal problem, Google, Yandex and other search engines, in fact, are also aggregators and not everyone is happy with what they are doing. And so, the information is posted for free access, you can process it without problems, but you can’t appropriate its authorship, it turns out that at least you need to indicate the sources, and then there are a lot of all sorts of features, pictures one thing, trademarks another, store prices third.