Nothing organized; it’s scattered piecemeal throughout the help docs.
The _ROWNUMBER column value is a unique numeric value assigned by AppSheet itself to each row. For data stored in a spreadsheet, the _ROWNUMBER value corresponds to the spreadsheet row number. I don’t know to what or even if the _ROWNUMBER value corresponds to anything for data stored in a database. For reasons I won’t bother with here, a _ROWNUMBER value may change when a sync occurs–which is why it shouldn’t be used as the key column–but between syncs, each row is guaranteed to have a unique, unchanging _ROWNUMBER value.
_THISROW is an alias for the key column of to the row for which an expression is being evaluated. If the expression being evaluated includes a SELECT() or related sub-expression,
[_THISROW] may be dereferenced from the sub-expression to access the evaluating row.
_THIS is an alias for the current column being subject to a column constraint. It’s just a shortcut.