Exceeded 5000 item limit in SharePoint

[JohnWickWithEraser]

Scenario: We exceeded the 5000 items limit in SharePoint in the threshold list.
Solution: we add indexes to existing columns (Title). But I don't know the limitation

1. How many items can be acquired when an index is set for a column with API filter?
   let items = await sp.web.lists.getById(this.listId).items.filter(Title eq '${title}').top(?) **** [?] is the number of items
   ....
2. How many items can be indexed? For example, we have a list with 20000 items. It's ok if we index all items. But I wonder if Indexing works when we have 3000000 items on the list.
   ....
3. If we have 3000000 items (index column Title). Then i use api
   sp.web.lists.getById(this.listId).items.filter(Title eq '${title}').top(5000). Will it work normally?
   ....