It is not done automatically by Softr.
It can be different values but I use 0 and 1.
0 for no index, 1 for index.
The Airtable field can be either a single line text field or a formula field (which is useful when you need to index - or not - automatically - some pages according to a specific condition (for example if the record has this => 1 (index true) but if the record has this =>0 (index false))
Here are two screenshots to showcase a very simple use case of SEO:Index, determined by a formula field.
In this use case, I want all open resources to be indexed and all closed resources to not be indexed.