Do not simultaneously purchase a huge number of links to a young site, because you need to simulate the natural growth of the link mass (at least a few pieces a week).
Actually, even for an age-old site, you should not buy thousands of links at the same time (especially in automatic mode), because you can fall under the “link explosion” filter, which will lead to their neglect by the search system.
Again, we try to imitate the naturalness of the process.
A commercial site should not participate in various link exchange systems.
Yes, and manual exchange is better to limit, and generally try to place less external links to other resources.
Free donor search without the use of any exchanges.
It is much more complicated and labor-consuming, but then with a high probability you will get a really working link if the donor site is thematic to yours and does not participate in the above mentioned exchange exchanges, and the link is located in the place where they will be clicked on.