[jozvolskyef]

In Czech all traffic must yield when entering a roundabout, including the tram. As far as I can tell the yield signs are there for foreigners might be used to different rules.

Regarding your second point, if you mean a car changing lanes from an inner lane to an outer lane, it does need to yield to everyone in that outer lane. This includes yielding to cyclists whose trajectory would intersect with the car. Again, the same applies to all traffic.