Is it common to have subway lines in Munich and Berlin accessible by escalator , or all steps? Elevators?
Like in the US ...Yes and No. Both Berlin and Munich have both S-Bahns and U-Bahns. Both systems in each city have stairs and elevators and escalators, if they are working.... Check out accessible travel for the systems for where you want to go; And, of course, when you want to travel since maintenance is an ongoing effort.
Examples:
https://sbahn.berlin/en/plan-a-journey/rail-stations/accessible-travel/
https://themunicheye.com/new-escalators-for-the-u-bahn-network-621
Just seconding what Nick said. It varies by station. If you absolutely need elevators or escalators, you will have to check the particular stations you are using.
Most German cities are working very hard to get all train stations, subway stations, and tram stations accessible, and many of them are doing pretty well in that respect by now.
However, that's not going to do you any good if you happen to end up having your hotel near a station that is currently under construction, so I can only agree with what the others said: Check out the current state of those stations that you will likely be using.