How does Santa get into houses that have no chimney?

Transporter beam.

Santa's a back door man.  Just leave the key under the mat or he may have to serve you with a warrant.

I paste this as a public service---it's a FAQ page.

https://www.santaanswers.com/017.html

Santa is real. so much  more real  that we are ghosts by comparison. and so are our walls and doors and everything else we call reality. Santa can walk right through all of it. The real question is how he manage to hold the socks steady while putting presents in them; Maybe he can adjust how real he (and the socks) are

Mom said she left the front door unlocked for him.  

Santa's magic door key.

"No problem!"