The objective of the technical SEO is to ensure that Google search engine is able to crawl and index your website appropriately. Search engine prefers a certain structure to the website to be able to crawl easily.
Google provides a webmaster guidlines at https://developers.google.com/search/docs/advanced/guidelines/overview
These guidelines are
1. Simple URL structure
2. Duplicate content should be avioded
3. Allow the links to be crawlable
4. Ensure that the Googlebot is not blocked
5. Ensure browser compatibility
Then there are content specific guidelines and quality specific guidelines
The other things that needs to be taken care of are
1. Your site speed – Check your site speed using google analytics
2. Check your website mobile compatibility
3. Look for crawl errors and address those with the help of your web developers
4. Upload the website maps
5. 404 erros on your website – What is 404 erros? A page which doesn’t on your website. How to address 404 error? Redirect it to relevant content which might be related to the page to which the consumers landed
To understand all these issues, the best way is to do a SEO Audit.
SEO Audit can be done using free tools or paid tools like semrush or Ahrefs.