How to run scripts on Ubuntu boot?

Question

linux

Good day.
There is Ubuntu 18.04, and there is a need to run the script at system startup. The system is server, so only ssh is available from the arsenal. Previously, rc.local was used for this, but on my system it no longer starts - compatibility fail. Googling, I found out that this is already the "old obsolete way".
Actually, the question is - what is the correct, modern, clean and native way to do the same thing that rc.local did? Without installing third-party packages (like upstart), to keep it simple and clear.
Thanks in advance.

Answer 1

about rc.local should already be forgotten as the day before yesterday.
and read about systemd units