Archimedes is known as the Father of Mathematics. It is one of the ancient sciences developed in time immemorial. Algebra, geometry, calculus and statistics & probability are considered to be the 4 main branches of mathematics. It is also broadly divided into pure mathematics and applied mathematics. Applied mathematics can be applied to real world problems.
Which is the toughest branch of mathematics? Geometry and trigonometry are both basic. Algebra can get very difficult at the university level, especially in graduate programs when you start to generalize concepts to abstract algebra and then explore commutative algebra. Alternatively, math gives us a way to understand patterns, to quantify relationships, and to predict the future. Math helps us understand the world and we use the world to understand math. The world is interconnected.
This post will further your knowledge in areas that all mathematicians will be interested in. Find out what resources you can read to learn more, and find a selection of useful links including a variety of ebooks, learning platforms, videos, tools and lecture notes on a wide array of topics, such as doing math foundation, set theory, logic, type theory, algebra and many many more.
Books

Calculus: Basic Concepts for High Schools
L.V. Tarasov 
Basics of Algebra, Topology, and Differential Calculus
Jean Gallier (University of Pennsylvania) 
Multivariable Calculus
G. Cain, J. Herod (Georgia Tech)  Wikibooks
 Online Mathematics Textbooks
 Beginning and Intermediate Algebra
General Resources
Learning Platforms
 Khan Academy
 Coursera
 MIT OpenCourseWare
 edX
 Brilliant
 WooTube
 Mathigon
 Calculus.org

Ximera
Free interactive mathematics textbooks (Ohio State University)
Questions and Answers
 Mathematics Stack Exchange

MathOverflow
For professional mathematicians
Encyclopedia
Youtube Series
Tools
 Symbolab
 Desmos
 Math Words
 Wolfram Alpha
 Maxima
 Sympy
 Sagemath
 GeoGebra
 Macaulay2
 Singular
 GNU Octave
 Magma
 Maple
 Matlab
 Wolfram Mathematica
 Free Math
Blogs

BetterExplained
Maintained by Kalid Azad 
ILoveMaths
For grades 6 thru 12 in K12 system 
3blue1brown
Animated Maths 
Mathsisfun
Simple text light weight site for students upto highschool 
MathematicsIsAScience
Peter Saveliev (Professor of mathematics at Marshall University, Huntington WV, USA)