The problem is with these two type defns in cypress.d.ts

// cypress.d.ts

/**
 * Get one or more DOM elements by selector.
 * The querying behavior of this command matches exactly how $(…) works in jQuery.
 * @see https://on.cypress.io/get
 * @example
 *    cy.get('.list>li')    // Yield the <li>'s in <.list>
 *    cy.get('ul li:first').should('have.class', 'active')
 *    cy.get('.dropdown-menu').click()
 */
get<E extends Node = HTMLElement>(
  selector: string, 
  options?: Partial<Loggable & Timeoutable & Withinable & Shadow>)
  : Chainable<JQuery<E>>

/**
 * Get one or more DOM elements by alias.
 * @see https://on.cypress.io/get#Alias
 * @example
 *    // Get the aliased ‘todos’ elements
 *    cy.get('ul#todos').as('todos')
 *    //...hack hack hack...
 *    //later retrieve the todos
 *    cy.get('@todos')
 */
get<S = any>(
  alias: string, 
  options?: Partial<Loggable & Timeoutable & Withinable & Shadow>)
  : Chainable<S>

You can "fix" it by reversing the order,

// cypress/support/index.ts

declare global {
  namespace Cypress {
    interface Chainable<Subject> {
      // change the order of these two dfns
      get<S = any>(alias: string, options?: Partial<Loggable & Timeoutable & Withinable & Shadow>): Chainable<S>
      get<E extends Node = HTMLElement>(selector: string, options?: Partial<Loggable & Timeoutable & Withinable & Shadow>): Chainable<JQuery<E>>
    }
  }
}

Now cy.get('@alias') types to any (because of get<S = any>), which is valid since aliases store any type.

But also the selector version is getting typed to any as well, it seems the identical type signatures make one of the defns redundant.

As Mikhail Bolotov points out, a different command name solves the issue, but I would define it as a custom command in /cypress/support/index.ts and call it getAlias since it has to work for any type.

AFAIK, it's impossible to derive the type from alias definition to the alias reference. But you can define it directly on the reference (works in a Typescript file only):

cy.get<number>('@userId').then(userId => // userId type is number });

You also can extract a plain function (or add a custom Cypress command) for such a number alias:

function cyNumber(alias: string) : Chainable<number> {
  return cy.get<number>(alias)
}

cyNumber('@userId').then(userId => // userId type is number });