-- Module: Mathlib.RingTheory.QuasiFinite.Basic
Goal:
R : Type u_1
S : Type u_2
inst : CommRing R
inst_1 : CommRing S
inst_2 : Algebra R S
p : Ideal R
inst_3 : p.IsPrime
q : Ideal S
inst_4 : q.IsPrime
inst_5 : q.LiesOver p
inst_6 : EssFiniteType R S
inst_7 : QuasiFiniteAt R q
e : (↑(Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
        {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }}) :
    Type u_2) ≃ₜ
  PrimeSpectrum ({ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }.asIdeal.Fiber S) :=
  PrimeSpectrum.preimageHomeomorphFiber R S { asIdeal := p, isPrime := ⋯ }
qF : PrimeSpectrum (p.Fiber S) :=
  (e :
      (a : (↑(PrimeSpectrum.comap (algebraMap R S) ⁻¹' {{ asIdeal := p, isPrime := ⋯ }}) : Type u_2)) →
        PrimeSpectrum ({ asIdeal := p, isPrime := ⋯ }.asIdeal.Fiber S))
    (Subtype.mk (p := fun (x : PrimeSpectrum S) ↦
      Membership.mem (γ := Set (PrimeSpectrum S))
        (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
          {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
        x)
      { asIdeal := q, isPrime := inst_4 } ⋯)
this_1 : QuasiFiniteAt p.ResidueField (S := p.Fiber S) qF.asIdeal
r : p.Fiber S
hr : r ∉ qF.asIdeal
hrq :
  (↑(PrimeSpectrum.basicOpen r) : Set (PrimeSpectrum (p.Fiber S))) =
    {{ asIdeal := qF.asIdeal, isPrime := ⋯ : PrimeSpectrum (p.Fiber S) }}
s : R
hs : s ∉ p
x : S
hsx : s • r = 1 ⊗ₜ[R] x
this :
  1 ⊗ₜ[R] x ∉
    RingHom.ker (F :=
      { asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }.asIdeal.ResidueField ⊗[R] S →+*
        (↑(Subtype.mk (p := fun (x : PrimeSpectrum S) ↦
                    Membership.mem (γ := Set (PrimeSpectrum S))
                      (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                        {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                      x)
                    { asIdeal := q, isPrime := inst_4 : PrimeSpectrum S } ⋯ :
                  { x : PrimeSpectrum S //
                    Membership.mem (γ := Set (PrimeSpectrum S))
                      (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                        {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                      x }) :
              PrimeSpectrum S).asIdeal.ResidueField)
      (AlgHom.toRingHom
        (TensorProduct.lift
            (Ideal.ResidueField.mapₐ { asIdeal := p, isPrime := ⋯ }.asIdeal
              (↑(Subtype.mk (p := fun (x : PrimeSpectrum S) ↦
                        Membership.mem (γ := Set (PrimeSpectrum S))
                          (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                            {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                          x)
                        { asIdeal := q, isPrime := inst_4 } ⋯ :
                      { x : PrimeSpectrum S //
                        Membership.mem (γ := Set (PrimeSpectrum S))
                          (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                            {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                          x }) :
                  PrimeSpectrum S).asIdeal
              (ofId R S) ⋯)
            (IsScalarTower.toAlgHom R S
              (↑(Subtype.mk (p := fun (x : PrimeSpectrum S) ↦
                          Membership.mem (γ := Set (PrimeSpectrum S))
                            (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                              {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                            x)
                          { asIdeal := q, isPrime := inst_4 : PrimeSpectrum S } ⋯ :
                        { x : PrimeSpectrum S //
                          Membership.mem (γ := Set (PrimeSpectrum S))
                            (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                              {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                            x }) :
                    PrimeSpectrum S).asIdeal.ResidueField)
            ⋯ :
          { asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }.asIdeal.ResidueField ⊗[R] S →ₐ[R]
            (↑(Subtype.mk (p := fun (x : PrimeSpectrum S) ↦
                        Membership.mem (γ := Set (PrimeSpectrum S))
                          (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                            {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                          x)
                        { asIdeal := q, isPrime := inst_4 : PrimeSpectrum S } ⋯ :
                      { x : PrimeSpectrum S //
                        Membership.mem (γ := Set (PrimeSpectrum S))
                          (Set.preimage (β := PrimeSpectrum R) (PrimeSpectrum.comap (algebraMap R S))
                            {{ asIdeal := p, isPrime := ⋯ : PrimeSpectrum R }})
                          x }) :
                  PrimeSpectrum S).asIdeal.ResidueField))
⊢ x ∉ q

Tactic:
simpa using this