If I have mobile internet(USB Stick to pick up internet from the nearest tower) does it have a constant speed or does it change area to area?

The speed differs from place to place. Consider the 'Line of Sight'. The tower should be nearer to you and the users on that particular area tower should be low. Because a constant amount of Bandwidth will be allocated to each area tower and when more number of users shares the same bandwidth, the speed eventually decreases. Hope this helps. :)

Short answer: It can change.

Long answer: The data throughput speed can change depending on the number of users sharing the available channels, their use, distance from the tower and a few other factors.

The "tower" will have a few antennas on it. Each antenna can deal with several frequencies/ranges. Depending on the carrier, each user could be doing voice, data (web surfing, downloading, watching video, uploading data or video, live streaming, etc.).

If everything remains the same - where "everything" = distance from the tower, no other users (only you), you are not moving, network infrastructure manufacturer (Nokia, Ericsson, Huawei, Cisco, Juniper, etc.), settings in all the network elements, antenna direction and down-tilt, etc., then your perception of the data throughput should be that it is constant. Change any variable and that perception of "constant" can change to variable speed.

We assume the service provider is not somehow controlling the data throughput for whatever reason and there is not some sort of network issue happening.