The short answer is Yes you can do SEO without hiring a marketing agency. SEO is a strategy that can be learned by anyone who spends time on that. There is a large amount of content available on Google and youtube for free where you can learn SEO by yourself free and can join classes which will cost you depending on where you study.

Read relevant article

https://webexpainer.wordpress.com/2021/09/23/what-is-a-landing-page-on-a-website/

https://batterseawebexpert.mystrikingly.com/blog/10-common-mistake-digital-marketers-make

https://batterseawebexpert.hpage.com/difference-between-dynamic-and-static-websites.html