This thesis deals with Panin and Walter's motivic spectrum BO. This spectrum is constructed using the real and quaternionic Grassmannians RG(r,n) and HGr(r,n) respectively, over schemes were 2 is invertible. We show that the construction of BO does not need the invertibility of 2. We also show that this spectrum is cellular over any base.

We give a model of restricted L_infinity-algebras in a nice preadditive symmetric monoidal infinity-category C as an algebra over the monad associated to an adjunction between C and the infinity-category of cocommutative bialgebras in C, where the left adjoint lifts the free associative algebra. If C is additive, we construct a canonical forgetful functor from restricted L_infinity-algebras in C to spectral Lie algebras in C and show that this functor is an equivalence if C is a Q-linear stable infinity-category. For every field K we construct a canonical forgetful functor from restricted L_infinity-algebras in connective K-module spectra to the infinity-category underlying a model structure on simplicial restricted Lie algebras over K.

In this dissertation we define an analogue, in the setting of infinity categories, of the classical notion of an enriched operad. We introduce six different models of enriched infinity operads. In particular, we generalize the operator category approach of Clark Barwick to the enriched setting as well as Moerdijk-Weiss' notion of dendroidal sets. The main part of the thesis consists of the comparison between different approaches to enriched operads.

We explore motivic homotopy theory over deeper bases than the spectrum of the integers: Starting from a commutative group object in a cartesian closed presentable infinity category, replacing the usual multiplicative group scheme in motivic spaces, we construct projective spaces, and show that infinite dimensional projective space is the classifying space of the group object. After passage to the stabilization, we construct a Snaith spectrum, calculate the cohomology represented by it for projective spaces and on its rationalization produce Adams operations and a splitting into summands of their eigenspaces.

This Ph.D. thesis deals with E1-ring structures on the Hermitian K-theory in the motivic setting, more precisely, the existence of such structures on the motivic spectrum representing the hermitianK-theory is proven. The presence of such structure is established through two different approaches. In both cases, we consider the category of algebraic vector bundles over a scheme, with the usual requirements to do motivic homotopy theory. This category has two natural symmetric monoidal structures given by the direct sum and the tensor product, together with a duality coming from the functor represented by the structural sheaf. The first symmetric monoidal structure is the one that we are going to group complete along this text, and we will see that the second one, the tensor product, is preserved giving rise to an E1-ring structure in the resulting spectrum. In the first case, a classic infinite loop space machine applies to the hermitian category of the category of algebraic vector bundles over a scheme. The second approach abords the construction using a new hermitian infinite loop space machine which uses the language of infinity categories. Both assemblies applied to our original category have like output a presheaf of E1-ring spectra. To get an spectrum representing the hermitian K-theory in the motivic context we need a motivic spectrum, i.e, a P1-spectrum. We use a delooping construction at the end of the text to obtain a presheaf of E1-ring P1-spectra from the two presheaves of E1-ring spectra indicated above.

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