Observation Details
Multiple elements are not using proper semantic HTML but instead are custom built, leading to numerous issues e.g. with keyboard accessibility (see 2.1.1 Keyboard). Including but not limited to, are the following elements:
Links ("Weiter ohne Gerät", Product cards, ...)
Buttons (Filter, Info icons, Footnote Asterisks)
Remediation Notes
Using native semantic HTML is always the preferred method.
If this is absolutely impossible in a situation, re-building can be viable but comes with dramatically increased time and effort to meet technical requirements. The BFIT-Bund Handreichung "Accessible design of user interface elements" shows technical requirements (e.g. keyboard navigation, mouse operation, etc.) for interactive elements. Looking into Accessible design of user interface elements – Links, quickly shows the complexity of what is required to meet the functionality, a properly used <a href="..."> element already includes.
Please (also) refer to observation 1.3.1 Info and Relationships – "Semantic HTML" for general remarks about the use of semantic HTML.