Left hand technique
Use weight/leverage more than muscle/tendon power to put fingers down. Remember, there are no muscles in the fingers, so the muscles that move your finger are in the arm near the elbow. If you use these too much, of course the hand is tense. If you press more than just the finger that is playing down, you are raising tension in the arm/hand/fingers and you are wasting the weight and leverage available. So, that’s what not to do and why.
So, what should you be doing instead? Think of the forearm, it has 2 bones in it. What does that mean for cello playing? It means that the forearm can rotate without any use of the upper arm or shoulder. When you play a note with the first finger, the forearm should rotate slightly clockwise (from the perspective of your elbow). This shifts the weight of the arm and hand toward the first finger. When you play a note with the 4th finger, you should rotate the hand somewhat counterclockwise, so that the weight of the arm goes into the 4th finger. Two things are accomplished by this, the motion of rotating the arm in the direction of the finger that will play releases the arm weight from the finger that was playing and shifts it to the finger that will be playing, and the motion itself moves the finger towards the string. As a result, more weight goes into the finger and the string (at the same time, tension is released from the other finger), and less effort is required to place the finger on the string.
Some cellists are unused to thinking about playing the cello this way, so you need to train yourself to think this way with slow, deliberate practice. Scale work is an excellent place to work on this, in addition to your practice of passagework. Another thing to bear in mind is that you do not need to press the string down into the fingerboard in order for the notes to sound clearly in most cases, especially the further you go from first position, as the string is most flexible in the middle.
Look at the video of Lynn Harrell in this link below at the 2 minute mark. He is demonstrating spiccato, but he makes a point about the pressure required for the string to sound.
Generally speaking, we choose fingerings to make playing a passage as simple and efficient as possible. Most western music is about scale-like melodies or implied harmony (arpeggiation). Knowing the patterns of the scales and arpeggios goes a long way into being able to recognize these patterns in pieces that we play and finger them accordingly. However, there are many times when playing a passage on one string has a much more homogenous effect that is desirable. The choice then of when to shift is sometimes merely practical, sometimes entirely musical.
There are a number of factors that effect the placement of fingers for playing fifths. The typical height of the A above the end of the fingerboard is 5.5mm, and the C is 8.5mm. The higher the pitch of the string, the shorter its transit is, the lower the pitch, the greater its transit. Transit means the complete distance the string travels in a single cycle that produces its pitch. Because of the difference in transit, all of the strings are a different height above the fingerboard, which means that if you press them down to the fingerboard at exactly the same distance from the nut on the different strings, you will distort the pitch of the highest string less than the lowest string, meaning the fifths will be out of tune if they are exactly parallel (the lower the string, the sharper it will be). In addition, each of your finger pads is a different shape and thickness, so this also influences what happens when you put your fingers down (especially if you are stopping a fifth with one finger). These things distort the relative location of the pitches, so it is never perfectly parallel from string to string.
Many cheap cellos also have necks/fingerboards that are too soft, which means the neck is unstable, making any double-stops unreliable. You need to experiment with these parameters in mind to find a solution that works for a particular passage.
We shift from position to position, not finger to finger. It takes time to develop reliable muscle memory and to build an effective mental map of the fingerboard. Conceptualizing the positions is very important. Timing of shifts and understanding the mechanics of shifting are important, and many great teachers have suggested that introducing the upper positions early is important.
Scales and Arpeggios
The best thing about scales and arpeggios is that they give you the opportunity to focus entirely on technical matters without “musical” considerations. Memorization, phrasing, analysis, etc. are not relevant. Arpeggios are all about learning the topography of the fingerboard, in every key. You have to cover a lot of territory, so how you shift (early and slow motion, no matter the tempo) is everything (ditto bow control of string crossings). You must train your concentration to control your body accurately in space and time, if your mind wanders, that tells you that you are not working on this correctly. When you are in performance, how well you have trained this mind/body connection is what is going to give you success, so you really must consider how you approach technical practice. Arpeggios are prominent features in standard concerto repertoire (Haydn D, Schumann, Brahms Double) and standard orchestral repertoire (Wagner, Strauss). This will matter a lot, if you work on it properly.