Using C# and CAML to query the list items in SharePoint - SPQuery

[@jpedrosant_]

Hello, sir!
Im a Brazilian, need help in one thing...
I need to create a CascadeDropdown, but i have 20k items on my list, the exibition limit is 5000, and my admin says that he can't expand it.
So, i have to create a filter that search what I type in the "patient" text field and work like the "relationshipListParentColumn" step in CascadeDropdowns.
Please, help me!

[@newabyuser]

You sir are an amazing, Please upload more videos

[@AlirezaAliabadi]

Thank you. More videos are on the way :)

[@yoshq7]

When I go to create a new Sharepoint project, it says:
The required version of SharePoint Foundation or SharePoint Server is not installed on this system. The target version of the SharePoint project is 2019.
And when I try to install Sharepoint Foundation 2013, it says:
Setup is unable to proceed due to the following error(s): This product requires Windows Server 2008 R2 Sp1 or above. Correct the issue(s) listed above and re-run setup.

[@mickyteddy7598]

Great sir Thanks!

[@MrMateortiz]

I need to filter by two columns as it would be, my initial calm query:
CamlQuery query = new CamlQuery();
query.ViewXml = "" +
"" +
"" +
"" +
"" +
"Pendiente" +
"" +
"" +
"" +
"";