Tier 4 general student visa and tier 4 student child visa have been abolished, and Part 3 student rules, such as for short term students and medics), have been deleted. The study routes as at Jan 2021 are student visa, child student visa, standard visitor visa and short term Study English in the UK visa. Study visas need a licenced student sponsor (to give you a CAS reference number), English, maintenance and NHS surcharge. There are special rules for medical courses.
Student visa lets you can stay a combined maximum of 5 years at degree level or 2 years below that. You can apply to extend if for a course that counts as academic progression. You might be able to switch to a student visa but not from a visit visa. It costs £348. It may allow you to bring dependents and to do limited work. The course has to be full time, and over 15 hours organised daytime study below degree level, or degree level, or part time for at least RQF 7 level, or an English course to at least CEFR B2. You need course fees and maintenance at £1,334 pcm in London or £1,023 elsewhere. You need English qualification such as SELT to at least CEFR B1 below degree level, or B2 for degree level (or whatever the college accepts, which may be higher).
Child student visa lets children attend independent schools for a course that is national curriculum, RQF or Ofsted etc for up to 6 years under 16 then 3 years over 16. You need an unconditional offer from a licenced child student sponsor who issues you with a CAS. It can include a short pre-sessional course but not a foundation course. You might be able to extend the visa or switch to it. It costs £348. Over 16s can work part time up to 10 hours a week, full time in holidays or up to 50% of the course on placement. It is not a route to settlement. Children under 12 can bring parents if parents successfully apply for a parent of a child student visa, otherwise for children over 12 parents can apply for a visitor visa for up to 6 months. You need the course fees, and maintenance of £1,560 for you and your parent or boarding fees instead or £570pcm for your settled carer or £1,334/£1,023 if you live independently, plus £625pcm per sibling who must also have a child student visa.
Visitor visa lets you study a course of under 6 months at an accredited institution, but not work. It costs £95. You cannot switch from it so would have to go home afterwards and apply for something else. It is also possible for research projects and medical electives. You need to prove you still live in your home country and will not overstay in the UK. You will be asked about your travel history, criminal record, money, whereabouts and employment.
Short term study English in the UK visa is for English courses 6-11 months long with an accredited institution such as a college with a student sponsor licence. It costs £186 and you need to prove money for course fees and maintenance. You cannot switch course, study, extend or bring dependents.