Would purchasing extra domains to promote a product be beneficial? For instance, a store that solely sells bicycles. In the case where we purchase supplementary domains with keywords such as bicycle, electro-velosiped.com, and moskov-velosiped.com and create 301 redirects to the primary site, would this positively impact search results?
Alternatively, do we need to develop a landing page for each domain with links to the main store?

There's really no point in doing it
Why not try SEO and aim for higher visibility on the main site?

Despite the fact that there are millions of people conducting both internal and external SEO, top positions are often already occupied by established players. Sometimes businesses will even create multiple websites in an attempt to push their competitors down in the rankings.

However, in your case, it seems like you haven't even made an effort towards SEO. Perhaps it's time to consider exploring this option in order to improve your online presence and attract more visitors to your website.

If you focus solely on the "bicycles" market, a single domain can generate substantial traffic, especially if the aim is to increase it.

To achieve this goal, there are less drastic methods such as diversifying your product range (not just the primary line, but also spare parts and accessories/related products) or broadening your semantic reach (e.g. informational articles).

As you have mentioned in previous responses, creating a landing page on the main site (be it on a subdomain or separate page) is a more straightforward and technically feasible approach.

To improve your website's search engine optimization, consider purchasing already-used domains that are relevant or closely related to your website's theme. These domains, known as "drops," can be easily integrated into your site through a 301 redirect, allowing you to gain the benefits of their established link profile.

By implementing this strategy, you can potentially boost your website's visibility and authority in search engine results pages. However, it's important to thoroughly research and vet any drops before purchasing them to ensure that they align with your website's goals and values.

Purchasing extra domains to promote a product can potentially be beneficial for improving search engine rankings and driving traffic to your website. However, it's important to approach this strategy carefully to ensure that it aligns with best practices and search engine guidelines.

Using exact-match domains with keywords related to your product, such as "bicycle" or "velosiped," can help improve your website's relevance for those keywords. Creating 301 redirects from supplementary domains to your primary site can potentially pass on some of the authority and relevance of those additional domains to your main website. This can have a positive impact on search results by improving the overall authority and trustworthiness of your website in the eyes of search engines.

Creating landing pages for each domain with links to the main store can also be an effective strategy. This approach allows you to create targeted content for specific keywords or locations, potentially improving the relevance and visibility of your website for those search terms.

It's important to note that while these strategies can be beneficial, they should be executed carefully to avoid potential pitfalls, such as thin or duplicate content. Additionally, prioritizing user experience and providing valuable, original content should always be a priority.
Success of these strategies will depend on the specific context of your business, the competitiveness of the keywords you're targeting, and the overall quality of your website and its content. It's recommended to consult with an experienced SEO professional to determine the best approach for your specific situation.