SQL Standard – What Does ‘ISO Standard Equivalent’ Mean?

If you look at the FROM clause syntax diagram you will see that there is only one place for the ON clause:

<joined_table> ::= 
{
    <table_source> <join_type> <table_source> ON <search_condition> 
    ...
}

What you find confusing is simple recursion, because <table_source> in <joined_table> above can be another <joined_table>:

[ FROM { <table_source> } [ ,...n ] ] 
<table_source> ::= 
{
    table_or_view_name ... 
    ...
    | <joined_table> 
    ...
}

To avoid confusion you should use parentheses in non-obvious cases (like your examples) to visually separate <table_sources>; they are not necessary for the query parser but useful for humans.

Dont try to reinvent the wheel as there is a cost to reinventing it.

Use Ola's maintenance scripts - they are tested well enough and follow best practice. You can even use LockTimeout in Ola's solution.

I would not worry on the fragmentation levels where the page count is less than 5K.

What is the best way to use rebuild\reorganize in this situation?

Do the index maintenance during less activity or maintenance time.