Data Lake : Data Catalog and Data Schema definitions
This has a regular structure ("schema"):

id  name   color  cost
1   dress  green    40
2   camera red     200
3   car    black 34000


Note the rigid "rows" and "columns".  The columns have specific datatypes" (string, number, etc).  Etc.

The following has some structure, but is also very open-ended and "self-defining".

{name: "dress"; size: "large"},
{name: "camera"; pixels: "12M"; zoom: "4x"},
{name: "car"; style: "sedan"},
{name: "family"; members: ["father", "mother", "children"]}


The former example is the key design of all RDBMSs.  It is the primary focus of MySQL, Postgres, etc.  It is designed and implemented for speed and scaling.  The "catalog" and "schema" define the structure separate from the data.

The latter example is less structured, less performant, but more flexible.  Perhaps this is the "data lake" you are asking about.

I suggest you focus on RDBMS and don't confuse yourself with "other" ideas.