......and how do you get the old interface back?

You can't, they got rid of the opt-out page.
Every popular social media website seems to do this, for some reason forcing the desktop interface to be designed to mobile standards.

I found a browser extension called "GoodTwitter" ( Firefox or Chrome ) that tricks twitter into giving you the old interface.

I'm still angry at my bank for converting web site to the mobile interface.

Sign of the times, phones are overtaking PCs. Many younger people don't even have or use PCs.

Where is it located, do you have a link?

Search Google on "GoodTwitter extension for _____"

Replace the blank with "firefox" or "chrome" depending on what browser you have.

So very annoying. Thanks for the tip. I went through similar pains with Instagram. I refuse to use it from the telephone but the price I have to pay is that I can comment but I cannot produce new posts unless I go through BlueStacks which has its own problems.