test.ts

describe('Home Page', () => {
  beforeEach(() => {
  })

  it('should have title', () => {
    cy.seedAndVisit();
    cy.contains('Angular (2+)');
  });

  it('should have a default article', () => {
    cy.seedAndVisit();
    cy.contains("Let's Get Started with Angular");
  });

  describe('search', () => {
    beforeEach(() => {
      
    })

    it('should search with real data', () => {
      cy.server();
      cy.visit('/');
      cy.get('input.search').type('night');
      cy.get('.search-button').click();
      cy.get('app-article').should('have.length', 1);
    })

    it('should filter to match when searching', () => {
      cy.seedAndVisit();
      cy.get('input.search').type('forms');
      cy.get('.search-button').click();
      cy.get('app-article').should('have.length', 1);
    })

  })

  describe('tags filtering', () => {
    
    it('should only show the one returned search article when a tag is selected', () => {
      cy.seedAndVisit(undefined, undefined, undefined, 'fixture:article2');
      cy.get('div.tags').eq(3).click();
      cy.get('app-article')
        .should('have.length', 1)
        .contains('Angular Forms FAQ');
    })

    it('should display all the related tags', () => {
      cy.seedAndVisit(undefined, undefined, undefined, 'fixture:article2');
      cy.get('div.tags').eq(3).click();
      cy.get('div.tags').as('tags').should('have.length', 3);
      cy.get('@tags').eq(0).contains('getting-started');
      cy.get('@tags').eq(1).contains('forms');
      cy.get('@tags').eq(2).contains('typescript');
    })

    it('should display the selected tag', () => {
      cy.seedAndVisit(undefined, undefined, undefined, 'fixture:article2');
      cy.get('div.tags').eq(3).click();
      cy.get('div.tags a.bold')
        .should('have.length', 1)
        .contains('getting-started');
    })
  })

})

1. empty beforeEach block
2. test 'should search with real data' calls cy.server() to start Cypress proxy but never defines any routes to spy or stub, so it does it for nothing
3. can't you use destructuring in the custom command rather than pass cy.seedAndVisit(undefined, undefined, undefined, 'fixture:article2')?
4. the selector names might be what you need like cy.get('div.tags') but they less than perfect I think. Our guide in https://docs.cypress.io/guides/references/best-practices.html#Selecting-Elements goes into details