Putting Asylum seekers aside, Immigration is needed to bring in the skills a country needds, while it can be argued a supermarket can find people locally to do the job of say shelf stacking when it comes to highly skilled work such as engineering, science and medicine (to name a few) the pool of skills is clearly much smaller. A company may employ an engineer from the UK to enhance their existing skills base. IIRC a small company in somerset UK provided the parachute for the latest mars rover, so that must at some point have required someone from the UK to work with the teams in the US (and elsewhere) on a visa.
So I hope that Trump has factored in, if you kick out the people with these specailist skills, you reduce competitiveness, but we also live in a global world where companies share expertise across international borders, it also helps improve cultural awareness and cohesion.