Legacy code is everywhere, It doesn't matter if the project code is there for 20 years or 5 years, if it's not maintainable, or if it doesn't follow a clear architecture, if there are not tests for it, then it's legacy.
Probably you already worked with one or more legacy project throughout your career, the following are the steps I take to deal with legacy code in my day-to-day work.
Delete everything that you don't need in that project, any empty directories, any files that are not used anywhere, any method that is not called anywhere, search your whole project for forgotten pieces that was useful one day but it's not needed anymore.
That include reviewing your dependencies and make sure all of them are used somewhere, if you're using some web framework that follows MVC make sure that all controllers are linked to a route, that it's not just sleeping there forgotten after removing a route.
I get the code coverage for the whole project and make a plan to cover the reset of it with tests, what's important for me is that the whole project runs in tests, that If I removed one part of these files a test should fail, it doesn't have to be unit test, but a test for the project features overall should exist, running everything under it.
Make sure all classes public interfaces are used outside of the class itself, if it's not used then hide it as a private method.
Rename methods and classes to follow some naming convention.
Move files around for better grouping, for example if you found a controller concern that should be a helper file then do it, if there is a model that should be a service then do it, small code movement without changing the business logic of your application.
The list goes on with small refactors, the gist is move stuff around without changing the business logic or making any risky changes.
Now that project structure is in place, write more unit tests, make sure that the units you have now are doing what it should do, exhaust your units logic at this step, you're trying to test every unit, use the black box kind of tests, don't bother yourself with implementation details, just give it some input and tests outputs and side effects, basically what the units should do not how it does it.
That include everything you love, from applying SOLID principles to splitting your monolith into separate micro services, it's all yours, but I keep in mind that I don't do all of it at the same time, I take one principle and apply it everywhere to the code, and if possible I write a test of a documentation page to make sure this rules is clean for other maintainers and it wouldn't be broken in the future, after I apply that principle I move on to the next one.
After the previous step your project becomes maintainable -hopefully- :) then the optimizations step comes in place, in that step I employ some caching layers, rewriting some parts so that it becomes faster, micro benchmarking and tiny optimizations.
It's always good to document whatever you applied to the project and make it easily accessible to other project maintainers, a wiki page is not enough, you have to give it the correct labels/tags, link it to the readme in the appropriate section, also a README file for every directory is a good idea, as it tells new maintainers why is this directory there and what should it he use it for and the rules governing that directory classes/modules.