How to promote a self-written site without an engine, without the ability to edit?

Question

Search Engine Optimization

The site is not supported, without cms, on the old self-written. It is impossible to make changes, but seo needs to be done. How to proceed?
Passed the test for employment, got this question. I worked in digital agencies, but somehow I didn’t have to deal with this.
Do I understand correctly that in order to promote such a site, it is necessary to work with links, crowd and other external sites? Or how is it done?

Answer 1

Assess the competitiveness of the environment. If the competitors are too strong, then do not take on the site at all. If there is a chance, then take up external optimization

Answer 2

If it is useful, informative and all so great - word of mouth.
If about dust bags:
Yandex advertising
Google advertising
Buying links
Spam
Social networks
And a super duper powerful system of the future TIKTOK! (sarcasm)
Did you interview Ashot? Who made a site, he did not want to pay and closed his access to the database or FTP? Sites without access do not exist. It "lies" somewhere (hosting, home server, etc.)

Answer 3

go back to 2001 and place links
in 2021 without reworking it for mobiles - alas

Answer 4

What do you mean measurements are not possible? Don't you have both hands or eyes there?
You just take it and make changes, at least if the site wants to make a profit, you migrate to the CMS with minimal losses