Hey ! To start working on building, you should start like me, on small buildings and then try to detail them more and more. Don’t start with big buildings.
For plugins I would recommend the gap full because sometimes in some spaces is better to fill them than to make another part and added in that small space that you have left.
To build learn some tutorials or practice, try making an easy build and then keep going, add as many details as you can do you can improve your skills.
Hope this is helpful!
Buildings just a lot of practice. Focus on simple builds and work your way up to advanced. It takes some time but at one point you can get very great at it.
Heres a Youtube channel you can take inspiration from, he makes speed builds and things to know about studio videos.