On the page where the categories are located, on the left there is a search - there you need to enter a keyword on which you want to find ads. The question is how to make it possible to find ads not on the first 3 letters (symbols) but in the whole in the ad suggestion?

For example: now if I have ads: I will sell a BMW car - then they can be found (such ads) only by the first 3 letters (symbols) ie, you need to enter where (keyword) enter not bmv as needed but the word sell - because the search works from the first 3 letters in the title of the ad. How can this be remedied? It is necessary that the search is also found on the main page of the site

@Hunter
There used to be in osclass docs search tunning. All depends on database.

At you on a demo site works - and at me on a site functions do not work. Can you help me with this? I can grant access to the site

All depends on database and configuration of fulltext search. Check osclass docs.

Here the search works fine (finds the word not only for the first three letters, symbols) -

And here, the search works poorly (to find only the first three letters, symbols) -

Honestly, I do not know how you can make the search on the 2nd picture work just as well as on 1 picture (that is, found not only the first 3 letters, symbols) Help me please solve this problem. I can give you all the data to enter the site.

Those are 2 completely different inputs.
Search works in way osclass and database supports it. For more information on that check osclass docs.

Why do you work on a demo site and I do not? What version of Osclass do you use on the demo site? and can such a difference depend on the version of Osсlass? Where can I find the documentation? And if it's not difficult - can you look at my site and tell me what's the problem?

The fact that my search does not work properly is a big problem. As search for a board is declared - that's all! I ask you to help me solve this problem.

The fact that my search does not work properly is a big problem. As search for a board is declared - that's all! I ask you to help me solve this problem.

1.add in your sql config file:
[mysqld]
ft_min_word_len=3
ft_max_word_len=10
2. save and restart sql  server

3. run sql query in phpadmin, with your table prefix...:
REPAIR TABLE os_t_item_description QUICK;
https://doc.osclass.org/Fine-Tuning_MySQL_Full-Text_Search_-_Improving_search