I have completed my Ph.D. in Mechanical Engineering (the interdisciplinary field of solar energy and applied mathematics) and have published some papers in which, although they have not been published in high-level journals, their good content made them receive an adequate number of citations. In your opinion, how can I get a postdoctoral job, given that I have already applied in several countries, several centers, and several universities?

Where I study there are more Ph.D. students in Engineering than any other discipline, its likely a similar situation in other places. There are so much more Ph.D. studentships being advertised than postdocs. You are up against a lot of competition. Ensure that you can evidence most if not all points in the person spec for the role advertised. Ask for feedback even if not shortlisted, some may give it.

Keep applying.

A typical (from experience) number of applicants to post in CS (admittedly not the same discipline) is about 20. This means applying 20 times, if you play the numbers game - which you should, considering you have no control over the other applicants.

Whilst doing so, I would keep an eye on industry, and not limit yourself to academic postdocs, unless you're really passionate about teaching and the conventional academic route. If it's research you're interested in, it's not always the case that an academic post (particularly in technological fields) is the best place to push the state-of-the-art, and it's often actually less viable than in a leading company (if you're in a leading solar energy provider, you'll likely have far more access to data, tech, and resources than a partner academic ever would). You will also earn considerably more, in all likelihood, if that's a consideration.

I applied for a postdoc late last year which I didn't get shortlisted for, I emailed to get feedback and was informed they had about 50! strong applicants and those shortlisted had several publications. Seems there are a lot of people competing for Engineering postdocs currently, the OP just needs to keep persevering and get feedback where they can.