I would like to find out and check keywords on competitors' websites. Could you please tell me how this can be done? And how can I use this information to my best advantage?

You can find out the keywords of your competitors' sites by using the keywords rank checker - https://pr-cy.io/tools/keywords-rank-checker/. This information will help you understand what queries users are most likely to use to find your competitors' site on Google. And if you manage this data properly, it will give you more opportunities to grow and get ahead of them.