According to the SQL standard, a "qualified join" (i.e. a join with ON or USING clause) should be parsed as follows:

<qualified join> ::=
  { <table reference> | <partitioned join table> }
  [ <join type> ] JOIN
  { <table reference> | <partitioned join table> }
  <join specification>

Where

<table reference> ::=
  <table factor>
| <joined table>

<table factor> ::=
  <table primary> [ <sample clause> ]

<table primary> ::=
  <table or query name>
| ...
| <parenthesized joined table>

As you can see, the right side of the JOIN operator is a <table reference>, which might itself be a <joined table>, and thus a <qualified join>, or a <parenthesized joined table>

This means, the following syntax is valid (and works, e.g. in SQL Server, resulting in a single record (1, 1, 1)) but is rejected by H2. The following syntax is putting a <parenthesized joined table> at the right side of X join:

select 
  X.A, 
  Y.B, 
  Z.C
from (
  select 1 A
) X
  join (
    (
      select 1 B
    ) Y
      join (
        select 1 C
      ) Z
      on Z.C = Y.B
  )
  on Y.B = X.A

Error message:

Syntax error in SQL statement "SELECT 
  X.A, 
  Y.B, 
  Z.C 
FROM ( 
  SELECT 1 A 
) X 
  JOIN ( 
    ( 
      SELECT 1 B 
    ) Y[*] 
      JOIN ( 
        SELECT 1 C 
      ) Z 
      ON Z.C = Y.B 
  ) 
  ON Y.B = X.A "; expected "UNION, MINUS, EXCEPT, INTERSECT, ORDER, OFFSET, FETCH, LIMIT, FOR, )"; ```

This works, putting a <table factor> at the right side of X join:

select 
  X.A, 
  Y.B, 
  Z.C
from (
  select 1 A
) X
  join
    (
      select 1 B
    ) Y
      join (
        select 1 C
      ) Z
      on Z.C = Y.B
  on Y.B = X.A